feat(sms): implement SMS messaging and metered billing

Implement a complete SMS messaging system including:
- Integration with SMS Manager.cz API for sending messages.
- Metered billing via Stripe using monthly aggregate invoice items.
- Backend services for managing SMS settings, usage logging, and monthly reporting.
- Database migrations for tenant settings, usage logs, and monthly reports.
- Frontend dashboard components for SMS configuration, usage tracking, and history.
- Support for customer phone numbers in the booking flow.

Includes new migrations, backend services, and frontend UI components.

apps/backend/migrations/00008_sms.sql

@@ -0,0 +1,47 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS tenant_sms_settings (
+  tenant_id uuid PRIMARY KEY REFERENCES tenants(id) ON DELETE CASCADE,
+  enabled boolean NOT NULL DEFAULT false,
+  sender_name text NOT NULL DEFAULT '',
+  monthly_limit integer,
+  stripe_subscription_item_id text,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE IF NOT EXISTS sms_usage_logs (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  tenant_id uuid NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+  recipient_phone text NOT NULL,
+  message_body text NOT NULL,
+  external_message_id text,
+  external_request_id text,
+  status text NOT NULL DEFAULT 'pending',
+  cost_cents integer NOT NULL DEFAULT 150,
+  sent_at timestamptz,
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE IF NOT EXISTS sms_monthly_reports (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  tenant_id uuid NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+  year_month text NOT NULL,
+  message_count integer NOT NULL DEFAULT 0,
+  total_cost_cents integer NOT NULL DEFAULT 0,
+  stripe_invoice_id text,
+  invoice_sent_at timestamptz,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  UNIQUE (tenant_id, year_month)
+);
+
+CREATE INDEX IF NOT EXISTS idx_sms_usage_tenant_month ON sms_usage_logs (tenant_id, date_trunc('month', created_at));
+CREATE INDEX IF NOT EXISTS idx_sms_usage_tenant_created ON sms_usage_logs (tenant_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_sms_reports_tenant ON sms_monthly_reports (tenant_id, year_month DESC);
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_sms_reports_tenant;
+DROP INDEX IF EXISTS idx_sms_usage_tenant_created;
+DROP INDEX IF EXISTS idx_sms_usage_tenant_month;
+DROP TABLE IF EXISTS sms_monthly_reports;
+DROP TABLE IF EXISTS sms_usage_logs;
+DROP TABLE IF EXISTS tenant_sms_settings;