REST API for Notes CRUD

## REST API for Notes CRUD

Extend the existing actix-web REST API with note-specific endpoints that integrate with the Note Engine module.

### New Endpoints

#### Notes CRUD

```
GET    /notes                    — List all notes (with pagination, prefix filter)
GET    /notes/{path}             — Get a single note by path
PUT    /notes/{path}             — Create or update a note
DELETE /notes/{path}             — Delete a note
POST   /notes/{path}/rename      — Rename a note (updates all indexes)
```

#### Note Relations

```
GET    /notes/{path}/backlinks   — List all notes linking TO this note
GET    /notes/{path}/links       — List all notes linked FROM this note
GET    /notes/{path}/graph       — Graph view data (new endpoint)
```

#### Tags

```
GET    /tags                     — List all tags
GET    /tags/{tag}/notes         — List notes with a specific tag (cursor pagination)
```

#### Search

```
GET    /search?q={query}&type=fulltext|tag|prefix
                                — Search notes by content, tag, or path prefix
```

#### Attachments

```
POST   /notes/{path}/attachments — Upload attachment (uses BlobStore)
GET    /notes/{path}/attachments/{name} — Download attachment
DELETE /notes/{path}/attachments/{name} — Remove attachment
```

#### Version History

```
GET    /notes/{path}/history     — List available versions
GET    /notes/{path}/history/{timestamp} — Get a specific version
POST   /notes/{path}/snapshot    — Create a manual snapshot
```

### Request/Response Formats

```json
// PUT /notes/my-note
{
  "content": "# My Note\n\nThis is a [[linked note]] with #tags",
  "frontmatter": {
    "title": "My Note",
    "aliases": ["MN"],
    "tags": ["reference"]
  }
}

// Response: GET /notes/my-note
{
  "path": "my-note",
  "content": "# My Note\n\nThis is a [[linked note]] with #tags",
  "frontmatter": { ... },
  "links": ["linked note"],
  "backlinks": ["other-note"],
  "tags": ["tags", "reference"],
  "created_at": "2026-05-25T10:00:00Z",
  "updated_at": "2026-05-25T11:30:00Z",
  "version": 3
}
```

### Auth Integration
- Use existing JWT/token auth middleware
- `Permission::Read` for GET endpoints
- `Permission::Write` for PUT/POST/DELETE
- `Permission::Admin` for version history management

### Acceptance Criteria
- [ ] All endpoints return correct HTTP status codes (200, 201, 404, 409)
- [ ] Note rename updates all link/backlink indexes
- [ ] Pagination works correctly for note listing
- [ ] Tag and prefix filters work
- [ ] Attachment upload/download works via BlobStore
- [ ] All endpoints are authenticated
- [ ] Integration tests for each endpoint

### Parent Epic
#275

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