feat: add database schema and backend foundation
Part 1 - Database (kanzlai schema in Supabase):
- Tenant-scoped tables: tenants, user_tenants, cases, parties,
deadlines, appointments, documents, case_events
- Global reference tables: proceeding_types, deadline_rules, holidays
- RLS policies on all tenant-scoped tables
- Seed: UPC proceeding types, 32 deadline rules (INF/CCR/REV/PI/APP),
ZPO civil rules (Berufung, Revision, Einspruch), 2026 holidays
Part 2 - Backend skeleton:
- config: env var loading (DATABASE_URL, SUPABASE_*, ANTHROPIC_API_KEY)
- db: sqlx connection pool with kanzlai search_path
- auth: JWT verification middleware adapted from youpc.org, context helpers
- models: Go structs for all tables with sqlx/json tags
- router: route registration with auth middleware, /health + placeholder API routes
- Updated main.go to wire everything together
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