refactor(db): set UTC timezone in pool config and add Querier docs

Set UTC timezone in pgxpool config to ensure consistent timestamp handling. Add detailed doc comment to Querier interface clarifying QueryRow vs Querier distinction.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/db/db.go

@@ -23,7 +23,12 @@ func Connect() error {
 		getEnv("POSTGRES_DB"),
 	)
 
-	pool, err := pgxpool.New(context.Background(), dsn)
+	poolCfg, err := pgxpool.ParseConfig(dsn)
+	if err != nil {
+		return err
+	}
+	poolCfg.ConnConfig.RuntimeParams["timezone"] = "UTC"
+	pool, err := pgxpool.NewWithConfig(context.Background(), poolCfg)
 	if err != nil {
 		return err
 	}