feat: Remove global db.Dbpool with dependency injection (Phase 0)

- Add Database struct in internal/db/database.go with Pool, Ctx, and RunMigrations()
- Update server.go to use Database struct with NewServerInstance()
- Add backend.go with InitBackend(), BackendRepo(), BackendCtx(), BackendPool()
- Update music.go and sync.go to use BackendRepo() and BackendCtx() instead of db.Dbpool/db.Ctx
- Update token_handler.go to accept pool parameter
- Update routes.go to use s.db.Pool for middleware
- Update cmd/main.go to use NewServerInstance() and HTTPServer()
- Update test_helpers.go to initialize backend with test database
- Update test files to use backend.BackendPool() and backend.BackendCtx()

Benefits:
- Easier to mock database for unit tests
- Follows Go best practices (dependency injection)
- Better architecture with explicit dependencies
- RunMigrations() replaces old Migrate_db() function

Note: Global db.Dbpool and db.Ctx still exist in dbHelper.go for backward compatibility
with test_helpers.go, but production code no longer uses them.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/server/routes.go

@@ -2,7 +2,6 @@ package server
 
 import (
 	"music-server/cmd/web"
-	"music-server/internal/db"
 	"music-server/internal/logging"
 	"music-server/internal/server/middleware"
 	"net/http"
@@ -126,7 +125,7 @@ func (s *Server) RegisterRoutes() http.Handler {
 
 	// Protected endpoints - require valid token
 	// Create token auth middleware with pool access
-	tokenAuthMiddleware := middleware.TokenAuthMiddleware(db.Dbpool)
+	tokenAuthMiddleware := middleware.TokenAuthMiddleware(s.db.Pool)
 
 	// Protected group with token authentication - will be used by VGMQ and Statistics API
 	_ = apiV1.Group("", tokenAuthMiddleware)