feat: add user AI budget override endpoints (#25439)

Implements https://linear.app/codercom/issue/AIGOV-285
Follow the structure established in
https://github.com/coder/coder/pull/25203

## Summary

Adds the `user_ai_budget_overrides` table and CRUD API at
`/api/v2/users/{user}/ai/budget`. An override sets a custom per-user
spend cap that supersedes group-budget resolution, attributing spend to
a specific group.

## Schema

```sql
CREATE TABLE user_ai_budget_overrides (
    user_id            UUID        PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
    group_id           UUID        NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
    spend_limit_micros BIGINT      NOT NULL CHECK (spend_limit_micros >= 0),
    created_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    updated_at         TIMESTAMPTZ NOT NULL DEFAULT NOW()
);
```

## Membership lifecycle

The membership invariant — a user must be a member of the attributed
group, including when that group is "Everyone" — would naturally be
expressed as a composite FK on `(user_id, group_id) →
group_members_expanded(user_id, group_id)`. PostgreSQL doesn't allow
foreign keys to reference views, so enforcement is split across two
mechanisms:

- **Write-time check.** A CHECK constraint on the table
(`user_ai_budget_overrides_must_be_group_member`) calls a `STABLE`
function `is_group_member(user_id, group_id)` that queries
`group_members_expanded`. The view surfaces both regular group
memberships and the implicit "Everyone" group memberships from
`organization_members`. Any INSERT or UPDATE that violates the predicate
is rejected with a Postgres `check_violation`, which the handler maps to
a 400. `is_group_member` is defined as a general predicate, reusable by
any future table that needs the same check.

- **Cascade on removal.** Two `BEFORE DELETE` triggers handle membership
loss:
- `trigger_delete_user_ai_budget_overrides_on_group_member_delete` on
`group_members` — covers regular group removals (admin action, OIDC
sync).
- `trigger_delete_user_ai_budget_overrides_on_org_member_delete` on
`organization_members` — covers the "Everyone" group, whose membership
lives in `organization_members`.

The single-column FKs on `users(id)` and `groups(id)` remain to cascade
on user or group deletion (those paths don't pass through
`group_members`).

## Authorization

The dbauthz layer gates each operation against the `User` and (for
writes) `Group` resources:

| Operation | User resource  | Group resource |
|-----------|----------------|----------------|
| `GET`     | `ActionRead`   | —              |
| `PUT`     | `ActionUpdate` | `ActionUpdate` |
| `DELETE`  | `ActionUpdate` | `ActionUpdate` |

For `DELETE`, the dbauthz layer fetches the existing override first to
learn the attributed `group_id`, then runs both checks.

### Role matrix

| Role         | GET | PUT | DELETE |
|--------------|-----|-----|--------|
| Owner        | ✅   | ✅   | ✅      |
| UserAdmin    | ✅   | ✅   | ✅      |
| OrgAdmin     | ✅   | ❌   | ❌      |
| OrgUserAdmin | ✅   | ❌   | ❌      |

Internal discussion:
https://codercom.slack.com/archives/C096PFVBZKN/p1779392747885359

## Audit logs
Audit logs will be addressed in a follow-up PR.

coderd/database/querier.go

@@ -198,6 +198,7 @@ type sqlcQuerier interface {
 	DeleteTailnetPeer(ctx context.Context, arg DeleteTailnetPeerParams) (DeleteTailnetPeerRow, error)
 	DeleteTailnetTunnel(ctx context.Context, arg DeleteTailnetTunnelParams) (DeleteTailnetTunnelRow, error)
 	DeleteTask(ctx context.Context, arg DeleteTaskParams) (uuid.UUID, error)
+	DeleteUserAIBudgetOverride(ctx context.Context, userID uuid.UUID) (UserAiBudgetOverride, error)
 	DeleteUserAIProviderKey(ctx context.Context, arg DeleteUserAIProviderKeyParams) error
 	DeleteUserAIProviderKeysByProviderID(ctx context.Context, aiProviderID uuid.UUID) error
 	DeleteUserChatCompactionThreshold(ctx context.Context, arg DeleteUserChatCompactionThresholdParams) error
@@ -738,6 +739,7 @@ type sqlcQuerier interface {
 	// inclusive.
 	GetTotalUsageDCManagedAgentsV1(ctx context.Context, arg GetTotalUsageDCManagedAgentsV1Params) (int64, error)
 	GetUnexpiredLicenses(ctx context.Context) ([]License, error)
+	GetUserAIBudgetOverride(ctx context.Context, userID uuid.UUID) (UserAiBudgetOverride, error)
 	GetUserAIProviderKeyByProviderID(ctx context.Context, arg GetUserAIProviderKeyByProviderIDParams) (UserAiProviderKey, error)
 	// GetUserAIProviderKeys is used by dbcrypt key rotation. Request paths should use
 	// user-scoped lookups instead of this bulk accessor.
@@ -1407,6 +1409,7 @@ type sqlcQuerier interface {
 	// used to store the data, and the minutes are summed for each user and template
 	// combination. The result is stored in the template_usage_stats table.
 	UpsertTemplateUsageStats(ctx context.Context) error
+	UpsertUserAIBudgetOverride(ctx context.Context, arg UpsertUserAIBudgetOverrideParams) (UserAiBudgetOverride, error)
 	// UpsertUserAIProviderKey preserves the original id and created_at when the
 	// user/provider pair already exists. On conflict, callers provide id and
 	// created_at for the insert path only.