feat!: scope chat model override settings to organizations (#28442)

Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

coderd/database/queries/users.sql

@@ -284,23 +284,6 @@ END
 WHERE user_configs.user_id = @user_id
 	AND user_configs.key = 'chat_debug_logging_enabled';
 
--- name: ListUserChatPersonalModelOverrides :many
-SELECT key, value FROM user_configs
-WHERE user_id = @user_id
-	AND key LIKE 'chat\_personal\_model\_override:%'
-ORDER BY key;
-
--- name: GetUserChatPersonalModelOverride :one
-SELECT value AS personal_model_override FROM user_configs
-WHERE user_id = @user_id
-	AND key = @key;
-
--- name: UpsertUserChatPersonalModelOverride :exec
-INSERT INTO user_configs (user_id, key, value)
-VALUES (@user_id::uuid, @key::text, @value::text)
-ON CONFLICT ON CONSTRAINT user_configs_pkey
-DO UPDATE SET value = @value::text;
-
 -- name: GetUserTaskNotificationAlertDismissed :one
 SELECT
 	value::boolean as task_notification_alert_dismissed