feat(media): storage for images and audio in notes (M1)

- Table media (migration 5), files in $DATA_DIR/media/<docId>/.
- POST /documents/{id}/media (write access), POST .../media/fetch copies an
  external https image (SSRF-guarded, not in the demo), GET /media/{id}
  with range requests for readers of a note that shows the file.
- Format detected from the content (no SVG); own sandbox CSP on answers.
- Blocks point to files through their content (media:<id>), so the note
  sync needs no change; compaction removes files no block mentions any
  more after a day. Demo reset and demo start clear the folder.
- Limits MEDIA_MAX_IMAGE_MB, MEDIA_MAX_AUDIO_MB, MEDIA_QUOTA_MB,
  MEDIA_DEMO_MAX_MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Doku/database.md

@@ -10,6 +10,7 @@ SQLite-Datei `mindsift.db` (Standard: `$DATA_DIR/mindsift.db`), geöffnet mit `j
 |---|---|---|
 | `users` | Konten | `email` (eindeutig), `username`, `password_hash`, `display_name`, `role` (`admin`/`member`), `is_active`, `token_version`, `alarm_channels` (kommagetrennt), `alarm_email`, `discord_webhook_url`, `custom_webhook_url`, `notify_before_minutes`, `avatar_url`, `organization_id` (leer = keine Organisation) |
 | `user_invites` | Einladungscodes | `code`, `email`, `role`, `organization_id`, `expires_at`, `used_by` |
+| `media` | Bilder und Audio in Notizen (Migration 5); Datei unter `$DATA_DIR/media/<doc_id>/<id>.<ext>` | `id`, `doc_id`, `owner_id`, `kind` (`image`/`audio`), `mime`, `size`, `source_url` (kopiertes externes Bild), `created_at` |
 | `sessions` | Angemeldete Geräte (Migration 4, ersetzt `revoked_tokens`) | `id` (= `jti` des Tokens), `user_id`, `created_at`, `last_used_at` (höchstens alle 5 Minuten aktualisiert), `user_agent`, `ip`, `revoked_at` (leer = aktiv) |
 | `user_themes` | Eigene CSS-Themes eines Benutzers | `user_id` (FK, `ON DELETE CASCADE`), `name`, `css_content` |
 | `api_keys` | Webhook-/API-Schlüssel | `key_prefix`, `key_hash` (SHA-256), `scopes` (nicht ausgewertet), `last_used_at` |