feat: document templates with auto-fill from case data (P1)
- Database: kanzlai.document_templates table with RLS policies
- Seed: 4 system templates (Klageerwiderung UPC, Berufungsschrift,
Mandatsbestätigung, Kostenrechnung)
- Backend: TemplateService (CRUD + render), TemplateHandler with
endpoints: GET/POST /api/templates, GET/PUT/DELETE /api/templates/{id},
POST /api/templates/{id}/render?case_id=X
- Template variables: case.*, party.*, tenant.*, user.*, date.*, deadline.*
- Frontend: /vorlagen page with category filters, template detail/editor,
render flow (select case -> preview -> copy/download), variable toolbar
- Quick action: "Schriftsatz erstellen" button on case detail page
- Also: resolved merge conflicts between audit-trail and role-based branches,
added missing Notification/AuditLog types to frontend
642877ae54