refactor(service): adopt feature-based architecture and rename pkg/diskcache

- Moved GORM/Redis CAPTCHA manager from internal/service/cap directly into the cohesive CAPTCHA app folder at internal/apps/cap/.
- Moved background system cleanup handler from internal/service/cleanup.go into internal/apps/upload/cleanup.go.
- Completely removed the global internal/service directory to keep module logic self-contained.
- Renamed the core utility engine pkg/diskcache to pkg/cache/disk to separate underlying utility code from db/config integrations.
- Renamed DiskCache struct in pkg/cache/disk to Cache to resolve revive package-name stuttering warning.
- Regenerated Swagger API documentation and confirmed all tests compile and pass with 0 linter issues.

internal/diskcache/cache.go

@@ -12,7 +12,7 @@ import (
 
 	"github.com/Rain-kl/Wavelet/internal/db"
 	"github.com/Rain-kl/Wavelet/internal/model"
-	pkgcache "github.com/Rain-kl/Wavelet/pkg/diskcache"
+	pkgcache "github.com/Rain-kl/Wavelet/pkg/cache/disk"
 )
 
 // Status represents the runtime cache statistics.
@@ -35,7 +35,7 @@ var ErrCacheMiss = pkgcache.ErrCacheMiss
 
 // DiskCache is a wrapper around the generic pkg/diskcache that integrates with the DB for configs.
 type DiskCache struct {
-	*pkgcache.DiskCache
+	*pkgcache.Cache
 }
 
 var (