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/version_handler.go

@@ -0,0 +1,39 @@
+package server
+
+import (
+	"net/http"
+
+	"music-server/internal/backend"
+
+	"github.com/labstack/echo/v5"
+)
+
+type VersionHandler struct{}
+
+func NewVersionHandler() *VersionHandler {
+	return &VersionHandler{}
+}
+
+// GetVersionHistory godoc
+//
+//	@Summary		Getting the version history of the backend
+//	@Description	get version history
+//	@Tags			version
+//	@Produce		json
+//	@Success		200	{array}	backend.VersionData
+//	@Router			/version/history [get]
+func (v *VersionHandler) GetVersionHistory(ctx *echo.Context) error {
+	return ctx.JSON(http.StatusOK, backend.GetVersionHistory())
+}
+
+// GetLatestVersion godoc
+//
+//	@Summary		Getting the latest version of the backend
+//	@Description	get latest version info
+//	@Tags			version
+//	@Produce		json
+//	@Success		200	{object}	backend.VersionData
+//	@Router			/version [get]
+func (v *VersionHandler) GetLatestVersion(ctx *echo.Context) error {
+	return ctx.JSON(http.StatusOK, backend.GetLatestVersion())
+}