feat(03-02): wire DATABASE_URL branching in main.go and fix cross-dialect UNIQUE detection

- Add DATABASE_URL env var branching: pgx/PostgreSQL when set, SQLite when absent
- Blank-import github.com/jackc/pgx/v5/stdlib to register 'pgx' driver
- Log 'Using PostgreSQL database' or 'Using SQLite database at {path}' on startup
- Replace RunMigrations with RunSQLiteMigrations (rename from Plan 01)
- Fix TagsHandler UNIQUE detection to use strings.ToLower for cross-dialect compat

cmd/diunwebhook/main.go

@@ -12,26 +12,39 @@ import (
 	"time"
 
 	diun "awesomeProject/pkg/diunwebhook"
+	_ "github.com/jackc/pgx/v5/stdlib"
 	_ "modernc.org/sqlite"
 )
 
 func main() {
-	dbPath := os.Getenv("DB_PATH")
-	if dbPath == "" {
-		dbPath = "./diun.db"
+	databaseURL := os.Getenv("DATABASE_URL")
+	var store diun.Store
+	if databaseURL != "" {
+		db, err := sql.Open("pgx", databaseURL)
+		if err != nil {
+			log.Fatalf("sql.Open postgres: %v", err)
+		}
+		if err := diun.RunPostgresMigrations(db); err != nil {
+			log.Fatalf("RunPostgresMigrations: %v", err)
+		}
+		store = diun.NewPostgresStore(db)
+		log.Println("Using PostgreSQL database")
+	} else {
+		dbPath := os.Getenv("DB_PATH")
+		if dbPath == "" {
+			dbPath = "./diun.db"
+		}
+		db, err := sql.Open("sqlite", dbPath)
+		if err != nil {
+			log.Fatalf("sql.Open sqlite: %v", err)
+		}
+		if err := diun.RunSQLiteMigrations(db); err != nil {
+			log.Fatalf("RunSQLiteMigrations: %v", err)
+		}
+		store = diun.NewSQLiteStore(db)
+		log.Printf("Using SQLite database at %s", dbPath)
 	}
 
-	db, err := sql.Open("sqlite", dbPath)
-	if err != nil {
-		log.Fatalf("sql.Open: %v", err)
-	}
-
-	if err := diun.RunMigrations(db); err != nil {
-		log.Fatalf("RunMigrations: %v", err)
-	}
-
-	store := diun.NewSQLiteStore(db)
-
 	secret := os.Getenv("WEBHOOK_SECRET")
 	if secret == "" {
 		log.Println("WARNING: WEBHOOK_SECRET not set — webhook endpoint is unprotected")

pkg/diunwebhook/diunwebhook.go

@@ -169,7 +169,7 @@ func (s *Server) TagsHandler(w http.ResponseWriter, r *http.Request) {
 		}
 		tag, err := s.store.CreateTag(req.Name)
 		if err != nil {
-			if strings.Contains(err.Error(), "UNIQUE") {
+			if strings.Contains(strings.ToLower(err.Error()), "unique") {
 				http.Error(w, "conflict: tag name already exists", http.StatusConflict)
 				return
 			}