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

backend/go.sum

@@ -0,0 +1,12 @@
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+github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
+github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
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+github.com/lib/pq v1.10.9/go.mod h1:AlVN5x4E4T544tWzH6hKfbfQvm3HdbOxrmggDNAPY9o=
+github.com/lib/pq v1.12.0 h1:mC1zeiNamwKBecjHarAr26c/+d8V5w/u4J0I/yASbJo=
+github.com/lib/pq v1.12.0/go.mod h1:/p+8NSbOcwzAEI7wiMXFlgydTwcgTr3OSKMsD2BitpA=
+github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=