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/time_entry.go

@@ -0,0 +1,24 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type TimeEntry 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"`
+	UserID          uuid.UUID  `db:"user_id" json:"user_id"`
+	Date            string     `db:"date" json:"date"`
+	DurationMinutes int        `db:"duration_minutes" json:"duration_minutes"`
+	Description     string     `db:"description" json:"description"`
+	Activity        *string    `db:"activity" json:"activity,omitempty"`
+	Billable        bool       `db:"billable" json:"billable"`
+	Billed          bool       `db:"billed" json:"billed"`
+	InvoiceID       *uuid.UUID `db:"invoice_id" json:"invoice_id,omitempty"`
+	HourlyRate      *float64   `db:"hourly_rate" json:"hourly_rate,omitempty"`
+	CreatedAt       time.Time  `db:"created_at" json:"created_at"`
+	UpdatedAt       time.Time  `db:"updated_at" json:"updated_at"`
+}