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/internal/models/document.go

@@ -0,0 +1,23 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Document struct {
+	ID          uuid.UUID        `db:"id" json:"id"`
+	TenantID    uuid.UUID        `db:"tenant_id" json:"tenant_id"`
+	CaseID      uuid.UUID        `db:"case_id" json:"case_id"`
+	Title       string           `db:"title" json:"title"`
+	DocType     *string          `db:"doc_type" json:"doc_type,omitempty"`
+	FilePath    *string          `db:"file_path" json:"file_path,omitempty"`
+	FileSize    *int             `db:"file_size" json:"file_size,omitempty"`
+	MimeType    *string          `db:"mime_type" json:"mime_type,omitempty"`
+	AIExtracted *json.RawMessage `db:"ai_extracted" json:"ai_extracted,omitempty"`
+	UploadedBy  *uuid.UUID       `db:"uploaded_by" json:"uploaded_by,omitempty"`
+	CreatedAt   time.Time        `db:"created_at" json:"created_at"`
+	UpdatedAt   time.Time        `db:"updated_at" json:"updated_at"`
+}