Remove obsolete UploadSessionManager and placeholder files from various domain modules to streamline the codebase and improve organization. Update documentation to reflect changes in file names.

Author: efraespada

…adio/api/service/UploadSessionManager.kt …/service/InMemoryUploadSessionManager.ktbackend/api/src/main/kotlin/com/vaultstadio/api/service/UploadSessionManager.kt renamed to backend/api/src/main/kotlin/com/vaultstadio/api/service/InMemoryUploadSessionManager.kt backend/api/src/main/kotlin/com/vaultstadio/api/service/UploadSessionManager.kt renamed to backend/api/src/main/kotlin/com/vaultstadio/api/service/InMemoryUploadSessionManager.kt

@@ -185,7 +185,7 @@ Existing Codecov config (`codecov.yml`) uses `range: "60..80"` and `threshold: 1
 
 | Component | File(s) | Suggested tests |
 |-----------|---------|------------------|
-| UploadSessionManager | `UploadSessionManager.kt` | Create session; add parts; complete; abort; expiry |
+| UploadSessionManager | `InMemoryUploadSessionManager.kt` | Create session; add parts; complete; abort; expiry |
 | ThumbnailCache | `ThumbnailCache.kt` | Get (hit/miss); put; invalidate; size/limits if applicable |
 | RouteExtensions | `RouteExtensions.kt` | Any extension functions used by routes (e.g. user/session extraction); test with mock call context |