docs(brain): update second brain with postgres multi-tenancy ADR

.brain/adr-log.md

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+# ADR 001: PostgreSQL 16 Hybrid Multi-Tenant Architektur
+
+## Status
+Akzeptiert
+
+## Kontext
+Gastro-SaaS Tischreservierung benötigt strenge Mandantentrennung bei hoher Performance.
+
+## Entscheidung
+- Nutzung von PostgreSQL 16 Schema-basierter Multi-Tenancy.
+- Master-Schema (`public` / `tenants` Tabelle) für Mandantenverwaltung.
+- Eigene Schemas pro Tenant (`tenant_<id>`) für operative Daten (`reservations`, `restaurant_tables`, `guests`).
+- Spring Boot 3 `SchemaBasedMultiTenantConnectionProvider` & `CurrentTenantIdentifierResolver`.
+
+## Konsequenzen
+- Hohe Datenisolation & einfache Backups pro Tenant.
+- Migrationen laufen schemaweise über Flyway.

.brain/index.md

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 # Second Brain Wiki
 
-- [Supabase Schema](file:///c:/source/webshop/.brain/supabase-schema.md)
+- [PostgreSQL Hybrid Multi-Tenant Setup](file:///c:/source/SmartSeat/SmartSeat/.brain/postgres-multi-tenant.md)
-- [ADR Log](file:///c:/source/webshop/.brain/adr-log.md)
+- [ADR Log](file:///c:/source/SmartSeat/SmartSeat/.brain/adr-log.md)
-- [LicServer API Spezifikation](file:///c:/source/webshop/.brain/licserver-api.md)
+- [Supabase Schema](file:///c:/source/SmartSeat/SmartSeat/.brain/supabase-schema.md)
+- [LicServer API Spezifikation](file:///c:/source/SmartSeat/SmartSeat/.brain/licserver-api.md)
+

.brain/postgres-multi-tenant.md

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+# PostgreSQL 16 Hybrid Multi-Tenant Setup
+
+## Architektur Overview
+- **Docker Setup**: PostgreSQL 16 Alpine via Docker Compose.
+- **Rollen-Isolation**: `app_user` (DML) & `migration_user` (DDL).
+- **Master Schema**: `tenants` Tabelle in `V1__init_master_schema.sql`.
+- **Tenant Schemas**: `dining_areas`, `restaurant_tables`, `guests`, `reservations`, `reservation_table_assignments`, `staff_users` in `V1__init_tenant_schema.sql`.
+
+## Backend Integration
+- `TenantContext` (ThreadLocal Schema Resolver)
+- `SchemaBasedMultiTenantConnectionProvider` (Hibernate Connection Handling)
+- HikariCP Connection Pool optimiert für Postgres in `application.yml`.