fix: resolve Docker volume permission denied on startup

Root cause: Docker named volumes are created as root, but the container
ran as the unprivileged 'oxicloud' user (UID 1001). Services like
thumbnail_service, image_transcode, and dedup_service call
create_dir_all under /app/storage during initialization, which fails
with 'Permission denied (os error 13)'.

Changes:
- Add entrypoint.sh that runs as root to chown /app/storage, then
  drops privileges via su-exec before executing the application
- Update Dockerfile to install su-exec, copy entrypoint, and use
  ENTRYPOINT instead of USER+CMD
- Downgrade id_mapping_service initial write failure from ERROR to WARN
  (empty in-memory map is perfectly valid, will persist on next save)
- Improve panic message in main.rs to hint at Docker permission issue

Fixes #<issue>

src/infrastructure/services/id_mapping_service.rs

@@ -181,11 +181,11 @@ impl IdMappingService {
             }
         }
         
-        // Write empty map to file
+        // Write empty map to file (best-effort: the in-memory map is valid even if disk write fails)
         match serde_json::to_string_pretty(&empty_map) {
             Ok(json) => {
                 if let Err(e) = fs::write(map_path, json).await {
-                    tracing::error!("Failed to write initial empty ID map: {}", e);
+                    tracing::warn!("Could not write initial empty ID map (will retry on next save): {}", e);
                 } else {
                     tracing::info!("Created initial empty ID map at {}", map_path.display());
                 }