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/dump.sql

@@ -257,7 +257,8 @@ CREATE TYPE api_key_scope AS ENUM (
     'ai_gateway_key:*',
     'ai_gateway_key:create',
     'ai_gateway_key:delete',
-    'ai_gateway_key:read'
+    'ai_gateway_key:read',
+    'ai_gateway_key:update'
 );
 
 CREATE TYPE app_sharing_level AS ENUM (