docs: document automatic migrations system

Updated CLAUDE.md to reflect new database migrations system:
- Added backend/migrations/ to directory structure
- Added schema_migrations table to database schema
- Updated deployment section with migration workflow
- Added reference to .claude/docs/technical/MIGRATIONS.md

The migrations system automatically applies SQL files (XXX_*.sql pattern)
on container startup, with tracking in schema_migrations table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -21,9 +21,11 @@ Teil der **Jinkendo**-App-Familie (人拳道). Domains: jinkendo.de / .com / .li
 backend/
 ├── main.py          # App-Setup + Router-Registration (~75 Zeilen)
 ├── db.py            # PostgreSQL Connection Pool
+├── db_init.py       # DB-Init + Migrations-System (automatisch beim Start)
 ├── auth.py          # Hash, Verify, Sessions, Feature-Access-Control
 ├── models.py        # Pydantic Models
 ├── feature_logger.py # Strukturiertes JSON-Logging (Phase 2)
+├── migrations/      # SQL-Migrationen (XXX_*.sql Pattern)
 └── routers/         # 14 Router-Module
     auth · profiles · weight · circumference · caliper
     activity · nutrition · photos · insights · prompts
@@ -83,6 +85,7 @@ frontend/src/
 ### v9c Finalisierung ✅
 - ✅ **Selbst-Registrierung:** POST /api/auth/register, E-Mail-Verifizierung, Auto-Login
 - ✅ **Trial-System UI:** Countdown-Banner im Dashboard (3 Urgency-Level)
+- ✅ **Migrations-System:** Automatische Schema-Migrationen beim Start (db_init.py)
 
 ### Offen v9d 🔲
 - Schlaf-Modul
@@ -121,6 +124,12 @@ Runner: Raspberry Pi (/home/lars/gitea-runner/)
 Manuell:
   cd /home/lars/docker/bodytrack[-dev]
   docker compose -f docker-compose[.dev-env].yml build --no-cache && up -d
+
+Migrations:
+  Werden automatisch beim Container-Start ausgeführt (db_init.py)
+  Nur nummerierte Dateien: backend/migrations/XXX_*.sql
+  Tracking in schema_migrations Tabelle
+  📚 Details: .claude/docs/technical/MIGRATIONS.md
 ```
 
 ## Datenbank-Schema (PostgreSQL 16)
@@ -142,10 +151,14 @@ subscriptions · coupons · coupon_redemptions · features
 tier_limits · user_feature_restrictions · user_feature_usage
 access_grants · user_activity_log
 
+Infrastruktur:
+schema_migrations – Tracking für automatische DB-Migrationen
+
 Feature-Logging (Phase 2):
 /app/logs/feature-usage.log  # JSON-Format, alle Feature-Zugriffe
 
 Schema-Datei: backend/schema.sql
+Migrationen: backend/migrations/*.sql (automatisch beim Start)
 ```
 
 ## API & Auth
@@ -233,6 +246,7 @@ Bottom-Padding Mobile: 80px (Navigation)
 | Backend-Architektur, Router, DB-Zugriff | `.claude/docs/architecture/BACKEND.md` |
 | Frontend-Architektur, api.js, Komponenten | `.claude/docs/architecture/FRONTEND.md` |
 | **Feature-Enforcement (neue Features hinzufügen)** | `.claude/docs/architecture/FEATURE_ENFORCEMENT.md` |
+| **Database Migrations (Schema-Änderungen)** | `.claude/docs/technical/MIGRATIONS.md` |
 | Coding Rules (Pflichtregeln) | `.claude/docs/rules/CODING_RULES.md` |
 | Lessons Learned (Fehler vermeiden) | `.claude/docs/rules/LESSONS_LEARNED.md` |
 | Feature Backlog (Übersicht) | `.claude/docs/BACKLOG.md` |