Restructure backend into services and fix security, crash and sync issues

Security
- Serve character images through os.Root so names like ../ can't escape
  the characters folder
- Sync start and /sync/reset are POST only (legacy and /api/v1)
- /api/v1/token/cleanup requires a token, password compared in constant
  time, tokens no longer logged
- Echo's rate limiter replaces the custom one; TRUSTED_PROXIES enables
  X-Forwarded-For for reverse proxies

Crashes and races
- Sync state lives in SyncService with atomics and a mutex instead of
  unsynchronised globals (fixes concurrent map writes during full sync)
- Only one sync can run; /api/v1 sync and music routes are locked during
  sync like the legacy ones
- No log.Fatal outside startup: errors become 4xx/5xx responses
- Search uses an in-order character match instead of unescaped regexes
- Empty queue / no songs return 404 instead of panicking or looping

Behaviour
- Sync result is computed once and stays the same between polls
- Swagger and /openapi serve the embedded spec
- Shutdown stops HTTP before closing the database
- Expired tokens are deleted with a context that outlives the request
- Files are served with Range support and correct content types; audio
  has no write deadline

Performance
- Migration 000006 adds soundtrack.fingerprint (names, sizes, mtimes) so
  unchanged folders are skipped without reading any files
- Songs are reconciled per soundtrack with one query plus needed writes;
  renames keep play counts
- Moved MUSIC_PATH updates soundtrack and song paths
- .id cleanup only runs for changed soundtracks or a full sync
- Classic random uses one SQL query; Gitea release lookups are cached
  with a timeout

Structure
- internal/config reads all environment variables once
- sqlc emits a Querier interface; backend globals become services
  injected into handlers
- One route registration shared by legacy and /api/v1
- Removed dead code (dbHelper.go, unused middleware, duplicate Gitea
  functions, embedded Swagger UI copy) and lib/pq
- templ and swag pinned as go tools

internal/server/middleware/sync_check.go

@@ -1,22 +1,22 @@
 package middleware
 
 import (
-	"music-server/internal/backend"
-	"music-server/internal/logging"
 	"net/http"
 
+	"music-server/internal/logging"
+
 	"github.com/labstack/echo/v5"
 )
 
-// SyncCheckMiddleware blocks requests when syncing is in progress.
-// It returns HTTP 423 Locked with a standard message.
-// This middleware should be applied to handlers that cannot execute during sync.
-func SyncCheckMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
-	return func(c *echo.Context) error {
-		if backend.Syncing {
-			logging.GetLogger().Info("Syncing is in progress - request blocked")
-			return c.JSON(http.StatusLocked, "Syncing is in progress")
+// SyncCheck blocks requests with HTTP 423 Locked while a sync is in progress.
+func SyncCheck(isSyncing func() bool) echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c *echo.Context) error {
+			if isSyncing() {
+				logging.GetLogger().Info("Syncing is in progress - request blocked")
+				return c.JSON(http.StatusLocked, "Syncing is in progress")
+			}
+			return next(c)
 		}
-		return next(c)
 	}
 }