feat(import): restore MindSift data and import Obsidian vaults

- Two separate paths: POST /import/mindsift (JSON backup or the Markdown
  archive) and POST /import/obsidian (vault ZIP into a folder named after it)
- Everything is planned and validated first; ?preview=1 returns what would
  be created and what is skipped (with reason), nothing is written
- MindSift data round-trips: folders, note structure, task lists with
  subtasks/due dates/priorities/notes, lists with quantity/category/done,
  triage thoughts from the JSON backup
- Markdown to outliner: nested lists, heading sections, callouts, code,
  paragraphs; front matter tags become hashtags; .obsidian/attachments skipped
- Limits against zip bombs and path tricks (100 MB upload, 10 MB per note,
  500 MB unpacked, 20k files, no .. escapes)
- Settings: two import cards with drag & drop, preview, confirm, result;
  touch-sized buttons; mobile download/upload verified

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

Doku/api/rest.md

@@ -34,6 +34,7 @@ Basis: `/api/v1`. Antworten sind JSON; Fehler haben die Form `{"error": "Text"}`
 | GET | `/export/markdown` | Obsidian-kompatibles ZIP (`mindsift-vault-DATUM.zip`), direkt gestreamt: Notizen als `.md` mit YAML-Frontmatter in der Ordnerstruktur des Nutzers (inkl. geteilter), `Aufgaben/<Liste>.md` im Format des Tasks-Plugins (`📅`, `⏫`/`🔺`), `Listen/<Liste>.md` |
 | GET/POST | `/themes` | Eigene CSS-Themes; anlegen oder mit `id` ändern (`name`, `css`, max. 200 KB) |
 | DELETE | `/themes/{id}` | Eigenes Theme löschen |
+| POST | `/import/mindsift`, `/import/obsidian` | Import (Multipart-Feld `file`, max. 100 MB): MindSift-JSON/-Archiv bzw. Obsidian-Vault-ZIP; `?preview=1` liefert nur die Zusammenfassung (`folders`, `notes`, `taskLists`, `tasks`, `lists`, `listItems`, `inbox`, `skipped`) |
 | GET/POST/DELETE | `/keys`, `/keys/{id}` | API-Keys verwalten (Klartext nur bei Erstellung) |
 
 ### Globale Suche (`GET /api/v1/search?q=`)