feat(coderd/database): add AI Gateway key auth lookup and last-used queries (#26505)

Adds DB methods`GetAIGatewayKeyIDByHashedSecret` and `UpdateAIGatewayKeyLastUsedAt`.
`GetAIGatewayKeyIDByHashedSecret` - returns AI Gateway key ID by hashed secret value.
`UpdateAIGatewayKeyLastUsedAt` - updates last used timestamp for given AI Gateway key. 
Used by standalone AI Gateway for authentication and keeping track of currently used keys.

coderd/database/migrations/000531_ai_gateway_key_update_scope.down.sql

@@ -0,0 +1,2 @@
+-- Enum additions to api_key_scope are intentionally not reverted because
+-- Postgres cannot drop enum values safely.

coderd/database/migrations/000531_ai_gateway_key_update_scope.up.sql

@@ -0,0 +1 @@
+ALTER TYPE api_key_scope ADD VALUE IF NOT EXISTS 'ai_gateway_key:update';