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/queries/ai_gateway_keys.sql

@@ -11,3 +11,19 @@ ORDER BY created_at ASC;
 -- name: DeleteAIGatewayKey :one
 DELETE FROM ai_gateway_keys WHERE id = $1
 RETURNING id, name, secret_prefix, created_at, last_used_at;
+
+-- name: GetAIGatewayKeyByHashedSecret :one
+-- Authenticates a standalone AI Gateway replica by its hashed key secret,
+-- returning the matched key. The lookup is an exact match on a unique index,
+-- so a returned row is itself proof the secret is valid.
+SELECT *
+FROM ai_gateway_keys
+WHERE hashed_secret = $1;
+
+-- name: UpdateAIGatewayKeyLastUsedAt :execrows
+-- Records liveness for an active Gateway DRPC session. The database sets the
+-- timestamp so it stays consistent regardless of clock drift between API
+-- replicas.
+UPDATE ai_gateway_keys
+SET last_used_at = NOW()
+WHERE id = $1;