feat: remove native chat usage limits in favor of AI Gateway budgets (#27329)

## Stack Context

This stack makes AI Gateway data and budgets the source of truth for AI
spend controls.

1. Re-back the per-chat cost endpoint with AI Gateway data (#27328,
merged).
2. **This PR:** remove native chat usage limits.
3. Remove native chat cost tracking and its dedicated admin UI (#27330).

## Summary

Removes the native usage-limit API, SDK types, SQL, and chat enforcement
for deployment, user, and group chat limits. Compact AI Gateway budget
indicators remain in the Agents sidebar, user menu, and group settings.
Gateway budget rejections and provider quota failures continue to
classify as usage-limit errors, including a 409 response for synchronous
title generation.

Budget-period labels now use the API's UTC boundaries, so users see the
same dates in every browser timezone. The documentation explains the AI
Gateway replacement, its licensing requirements, and the differences
from native limits.

No schema is dropped in this release. The usage-limit table, index, user
and group columns, constraints, audit mappings, and generated scan
fields remain for mixed-version rolling upgrades. #27600 tracks their
removal after the compatibility window.

## Breaking change

Native day, week, and month chat spend limits are removed and are not
migrated. AI Gateway budgets are month-based, group-scoped with per-user
overrides, and require the AI Gateway entitlement. Deployments without
that entitlement no longer have chat spend enforcement.

> Mux prepared this PR on Mike's behalf.

coderd/database/queries/chats.sql

@@ -2434,55 +2434,6 @@ LIMIT
 OFFSET
     sqlc.arg('page_offset')::int;
 
--- name: GetChatUsageLimitConfig :one
-SELECT * FROM chat_usage_limit_config WHERE singleton = TRUE LIMIT 1;
-
--- name: UpsertChatUsageLimitConfig :one
-INSERT INTO chat_usage_limit_config (singleton, enabled, default_limit_micros, period, updated_at)
-VALUES (TRUE, @enabled::boolean, @default_limit_micros::bigint, @period::text, NOW())
-ON CONFLICT (singleton) DO UPDATE SET
-    enabled = EXCLUDED.enabled,
-    default_limit_micros = EXCLUDED.default_limit_micros,
-    period = EXCLUDED.period,
-    updated_at = NOW()
-RETURNING *;
-
--- name: ListChatUsageLimitOverrides :many
-SELECT u.id AS user_id, u.username, u.name, u.avatar_url,
-       u.chat_spend_limit_micros AS spend_limit_micros
-FROM users u
-WHERE u.chat_spend_limit_micros IS NOT NULL
-ORDER BY u.username ASC;
-
--- name: UpsertChatUsageLimitUserOverride :one
-UPDATE users
-SET chat_spend_limit_micros = @spend_limit_micros::bigint
-WHERE id = @user_id::uuid
-RETURNING id AS user_id, username, name, avatar_url, chat_spend_limit_micros AS spend_limit_micros;
-
--- name: DeleteChatUsageLimitUserOverride :exec
-UPDATE users SET chat_spend_limit_micros = NULL WHERE id = @user_id::uuid;
-
--- name: GetChatUsageLimitUserOverride :one
-SELECT id AS user_id, chat_spend_limit_micros AS spend_limit_micros
-FROM users
-WHERE id = @user_id::uuid AND chat_spend_limit_micros IS NOT NULL;
-
--- name: GetUserChatSpendInPeriod :one
--- Returns the total spend for a user in the given period.
--- When organization_id is NULL, spend across all organizations is
--- returned (global behavior). Otherwise only spend within the
--- specified organization is included.
-SELECT COALESCE(SUM(cm.total_cost_micros), 0)::bigint AS total_spend_micros
-FROM chat_messages cm
-JOIN chats c ON c.id = cm.chat_id
-WHERE c.owner_id = @user_id::uuid
-  AND (sqlc.narg('organization_id')::uuid IS NULL
-       OR c.organization_id = sqlc.narg('organization_id')::uuid)
-  AND cm.created_at >= @start_time::timestamptz
-  AND cm.created_at < @end_time::timestamptz
-  AND cm.total_cost_micros IS NOT NULL;
-
 -- name: GetTotalChatMessageRuntimeMsInRange :one
 -- Computes hb_agent_runtime_v1 usage event payloads. Deliberately includes
 -- soft-deleted messages and messages from all chats.
@@ -2508,49 +2459,6 @@ WHERE enabled = TRUE
     )
   );
 
--- name: ListChatUsageLimitGroupOverrides :many
-SELECT
-    g.id AS group_id,
-    g.name AS group_name,
-    g.display_name AS group_display_name,
-    g.avatar_url AS group_avatar_url,
-    g.chat_spend_limit_micros AS spend_limit_micros,
-    (SELECT COUNT(*)
-        FROM group_members_expanded gme
-        WHERE gme.group_id = g.id
-          AND gme.user_is_system = FALSE) AS member_count
-FROM groups g
-WHERE g.chat_spend_limit_micros IS NOT NULL
-ORDER BY g.name ASC;
-
--- name: UpsertChatUsageLimitGroupOverride :one
-UPDATE groups
-SET chat_spend_limit_micros = @spend_limit_micros::bigint
-WHERE id = @group_id::uuid
-RETURNING id AS group_id, name, display_name, avatar_url, chat_spend_limit_micros AS spend_limit_micros;
-
--- name: DeleteChatUsageLimitGroupOverride :exec
-UPDATE groups SET chat_spend_limit_micros = NULL WHERE id = @group_id::uuid;
-
--- name: GetChatUsageLimitGroupOverride :one
-SELECT id AS group_id, chat_spend_limit_micros AS spend_limit_micros
-FROM groups
-WHERE id = @group_id::uuid AND chat_spend_limit_micros IS NOT NULL;
-
--- name: GetUserGroupSpendLimit :one
--- Returns the minimum (most restrictive) group limit for a user.
--- Returns -1 if no group limits match the specified scope.
--- When organization_id is NULL, groups across all organizations are
--- considered (global behavior). Otherwise only groups within the
--- specified organization are considered.
-SELECT COALESCE(MIN(g.chat_spend_limit_micros), -1)::bigint AS limit_micros
-FROM groups g
-JOIN group_members_expanded gme ON gme.group_id = g.id
-WHERE gme.user_id = @user_id::uuid
-  AND (sqlc.narg('organization_id')::uuid IS NULL
-       OR g.organization_id = sqlc.narg('organization_id')::uuid)
-  AND g.chat_spend_limit_micros IS NOT NULL;
-
 -- name: GetChatsByWorkspaceIDs :many
 SELECT *
 FROM chats_expanded
@@ -2558,44 +2466,6 @@ WHERE archived = false
   AND workspace_id = ANY(@ids::uuid[])
 ORDER BY workspace_id, updated_at DESC;
 
--- name: ResolveUserChatSpendLimit :one
--- Resolves the effective spend limit for a user using the hierarchy:
--- 1. Individual user override (highest priority, applies globally across
---    all organizations since it lives on the users table)
--- 2. Minimum group limit across the user's groups
--- 3. Global default from config
--- Returns -1 if limits are not enabled.
--- When organization_id is NULL, groups across all organizations are
--- considered (global behavior). Otherwise only groups within the
--- specified organization are considered.
--- limit_source indicates which tier won: 'user', 'group', 'default',
--- or 'disabled'.
-SELECT CASE
-    WHEN NOT cfg.enabled THEN -1
-    WHEN u.chat_spend_limit_micros IS NOT NULL THEN u.chat_spend_limit_micros
-    WHEN gl.limit_micros IS NOT NULL THEN gl.limit_micros
-    ELSE cfg.default_limit_micros
-END::bigint AS effective_limit_micros,
-CASE
-    WHEN NOT cfg.enabled THEN 'disabled'
-    WHEN u.chat_spend_limit_micros IS NOT NULL THEN 'user'
-    WHEN gl.limit_micros IS NOT NULL THEN 'group'
-    ELSE 'default'
-END AS limit_source
-FROM chat_usage_limit_config cfg
-CROSS JOIN users u
-LEFT JOIN LATERAL (
-    SELECT MIN(g.chat_spend_limit_micros) AS limit_micros
-    FROM groups g
-    JOIN group_members_expanded gme ON gme.group_id = g.id
-    WHERE gme.user_id = @user_id::uuid
-      AND (sqlc.narg('organization_id')::uuid IS NULL
-           OR g.organization_id = sqlc.narg('organization_id')::uuid)
-      AND g.chat_spend_limit_micros IS NOT NULL
-) gl ON TRUE
-WHERE u.id = @user_id::uuid
-LIMIT 1;
-
 -- name: UpdateChatLastReadMessageID :exec
 -- Updates the last read message ID for a chat. This is used to track
 -- which messages the owner has seen, enabling unread indicators.