feat: time tracking + billing — hourly rates, time entries, invoices (P1)

Database: time_entries, billing_rates, invoices tables with RLS.
Backend: CRUD services+handlers for time entries, billing rates, invoices.
  - Time entries: list/create/update/delete, summary by case/user/month
  - Billing rates: upsert with auto-close previous, current rate lookup
  - Invoices: create with auto-number (RE-YYYY-NNN), status transitions
    (draft->sent->paid, cancellation), link time entries on invoice create
API: 11 new endpoints under /api/time-entries, /api/billing-rates, /api/invoices
Frontend: Zeiterfassung tab on case detail, /abrechnung overview with filters,
  /abrechnung/rechnungen list+detail with status actions, billing rates settings
Also: resolved merge conflicts between audit-trail and role-based branches,
  added missing types (Notification, AuditLogResponse, NotificationPreferences)

backend/internal/models/billing_rate.go

@@ -0,0 +1,18 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type BillingRate struct {
+	ID        uuid.UUID  `db:"id" json:"id"`
+	TenantID  uuid.UUID  `db:"tenant_id" json:"tenant_id"`
+	UserID    *uuid.UUID `db:"user_id" json:"user_id,omitempty"`
+	Rate      float64    `db:"rate" json:"rate"`
+	Currency  string     `db:"currency" json:"currency"`
+	ValidFrom string     `db:"valid_from" json:"valid_from"`
+	ValidTo   *string    `db:"valid_to" json:"valid_to,omitempty"`
+	CreatedAt time.Time  `db:"created_at" json:"created_at"`
+}