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.

AGENTS.md

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+# Bookra Project Notes
+
+## Build & Test Commands
+- `npm run build:frontend` — Build SolidJS frontend
+- `npm run build:backend` — Build Go backend
+- `npm run test` — Run backend tests
+- `npm run verify` — Full verification (client gen, lint, test, build)
+
+## Database
+- Uses PostgreSQL via Neon (pooled URL for app, direct URL for migrations)
+- Migrations with Goose: `npm run db:migrate:up`
+- SQLC for typed queries: `npm run db:generate`
+
+## SMS Feature
+
+### Architecture
+- Optional add-on, off by default
+- Only available on **Pro** and **Business** plans
+- Uses SMS Manager.cz JSON API v2 (`https://api.smsmngr.com/v2`)
+- Metered billing via Stripe (1.50 CZK per SMS)
+- Tracks usage locally in `sms_usage_logs` table
+
+### Database Tables
+- `tenant_sms_settings` — per-tenant SMS config (enabled, sender, limit, stripe item ID)
+- `sms_usage_logs` — every SMS sent with cost tracking
+- `sms_monthly_reports` — aggregated monthly usage for invoices
+
+### API Endpoints
+- `GET /v1/sms/settings` — Get SMS settings + current month stats
+- `POST /v1/sms/settings` — Enable/disable SMS, configure sender/limit
+- `POST /v1/sms/send` — Send an SMS (tracked & billed)
+- `GET /v1/sms/usage` — Usage for a specific month
+- `GET /v1/sms/history` — Recent SMS logs
+- `GET /v1/sms/invoices` — Monthly invoice reports
+- `POST /v1/internal/jobs/sms/invoices` — Cron endpoint to generate monthly reports & emails
+
+### What to Configure on Stripe
+1. **Create standard Prices** for SMS in each currency:
+   - Product: "SMS Messages"
+   - Price: 1.50 CZK per unit (or equivalent in USD/EUR)
+   - Billing mode: **Standard** one-time (not metered)
+   - No free trial
+
+2. **Environment variables** to add:
+   ```
+   BOOKRA_STRIPE_SMS_CZK_PRICE_ID=price_xxx
+   BOOKRA_STRIPE_SMS_USD_PRICE_ID=price_yyy
+   BOOKRA_STRIPE_SMS_EUR_PRICE_ID=price_zzz
+   BOOKRA_SMSMANAGER_API_KEY=your_smsmanager_api_key
+   ```
+
+### Stripe CLI Commands (for testing)
+```bash
+# Login to Stripe
+stripe login
+
+# Create test product
+stripe products create --name="SMS Messages"
+
+# Create prices in each currency (replace prod_xxx with actual product ID)
+stripe prices create --product=prod_xxx --unit-amount=150 --currency=czk
+stripe prices create --product=prod_xxx --unit-amount=6 --currency=usd
+stripe prices create --product=prod_xxx --unit-amount=6 --currency=eur
+
+# Listen to webhooks locally
+stripe listen --forward-to http://localhost:8080/v1/webhooks/stripe
+```
+
+### Monthly Invoice Flow
+- At month end, a background job (`POST /v1/internal/jobs/sms/invoices`) aggregates all SMS usage per tenant
+- It creates a Stripe `invoiceitem` with quantity = messages sent × 1.50 CZK (or configured currency price)
+- The item is added to the customer's next subscription invoice automatically
+- A usage summary email is sent showing: messages sent, total cost, and invoice details
+- Reports are visible in-app under Settings > SMS Messages > Invoice reports
+
+### Taxes
+- The 1.50 CZK is the base unit price
+- Stripe handles tax calculation based on the customer's location and your tax settings
+- Displayed prices in the app show pre-tax amounts; the final invoice includes tax
+
+### No Free Trial
+- SMS is charged from the first message
+- No trial period — usage is aggregated and invoiced monthly