feat: add background_tasks + list_snapshots tools (v0.3.4)

- background_tasks: SYNO.FileStation.BackgroundTask::list (v3) — paginated
  table of active/recent copy/move/delete/extract/compress tasks
- list_snapshots: SYNO.FileStation.Snapshot::list (v2) — Btrfs snapshots
  per share; maps error 400 to a clear Btrfs-required message
- 20 new tests (107 total)
- SPEC.md and CLAUDE.md updated (26 tools)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -146,7 +146,7 @@ src/mcp_synology_filestation/
     └── filestation.py   # register_filestation(mcp, config, client)
 ```
 
-## Implemented Tools (v0.2.10 — 20 tools)
+## Implemented Tools (v0.3.4 — 26 tools)
 
 | Tool                  | Description                                                  |
 |-----------------------|--------------------------------------------------------------|
@@ -170,17 +170,24 @@ src/mcp_synology_filestation/
 | `create_sharing_link` | Create a public sharing link (optional password + expiry)    |
 | `list_sharing_links`  | List all sharing links (paginated table)                     |
 | `delete_sharing_link` | Delete a sharing link by ID                                  |
+| `get_thumbnail`       | Fetch a thumbnail for an image/video file as base64          |
+| `list_favorites`      | List all FileStation user favorites                          |
+| `add_favorite`        | Pin a path as a FileStation favorite                         |
+| `delete_favorite`     | Remove a path from FileStation favorites                     |
+| `background_tasks`    | List FileStation background tasks (copy, move, delete, etc.) |
+| `list_snapshots`      | List Btrfs snapshots for a share (requires Btrfs volume)     |
 
 See [SPEC.md](SPEC.md) for full tool specifications and DSM call details.
 
+## DSM Quirks (continued)
+
+- **`SYNO.FileStation.Snapshot::list`:** Returns error 400 when the share is not on a
+  Btrfs-formatted volume. Confirmed: this NAS has no Btrfs volumes — `list_snapshots`
+  maps error 400 to a clear "requires Btrfs-formatted volume" message.
+- **`SYNO.FileStation.BackgroundTask`:** Only the `list` method is available (v1–v3).
+  No stop/cancel/clear methods exist on this firmware.
+
 ## Roadmap
 
-### v0.3 — next
-| Tool               | API                              |
-|--------------------|----------------------------------|
-| `get_thumbnail`    | `SYNO.FileStation.Thumb`         |
-| `list_favorites`   | `SYNO.FileStation.Favorite`      |
-| `add_favorite`     | `SYNO.FileStation.Favorite`      |
-| `delete_favorite`  | `SYNO.FileStation.Favorite`      |
-| `list_snapshots`   | `SYNO.FileStation.Snapshot`      |
-| `background_tasks` | `SYNO.FileStation.BackgroundTask`|
+### v0.4 — candidates
+No further tools currently planned.