Update README with MQTT tables and parent_user_id documentation

Added documentation for:
- parent_user_id column in User table (hierarchical user management)
- mqtt_credentials table with device authentication
- mqtt_acl_rules table for topic permissions
- mqtt_sync_status table for broker synchronization
- All new database indexes
- Updated ER diagram with new relationships

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -158,6 +158,7 @@ username        TEXT UNIQUE NOT NULL
 email           TEXT
 passwordHash    TEXT NOT NULL
 role            TEXT NOT NULL DEFAULT 'VIEWER'  -- ADMIN oder VIEWER
+parent_user_id  TEXT                            -- FK zu User.id (Hierarchie)
 createdAt       TEXT DEFAULT (datetime('now'))
 updatedAt       TEXT DEFAULT (datetime('now'))
 lastLoginAt     TEXT
@@ -176,10 +177,44 @@ createdAt       TEXT DEFAULT (datetime('now'))
 updatedAt       TEXT DEFAULT (datetime('now'))
 ```
 
+**MQTT Credentials Tabelle:**
+```sql
+id                  INTEGER PRIMARY KEY AUTOINCREMENT
+device_id           TEXT NOT NULL UNIQUE            -- FK zu Device.id
+mqtt_username       TEXT NOT NULL UNIQUE
+mqtt_password_hash  TEXT NOT NULL
+enabled             INTEGER DEFAULT 1               -- 0 oder 1
+created_at          TEXT DEFAULT (datetime('now'))
+updated_at          TEXT DEFAULT (datetime('now'))
+```
+
+**MQTT ACL Rules Tabelle:**
+```sql
+id              INTEGER PRIMARY KEY AUTOINCREMENT
+device_id       TEXT NOT NULL                   -- FK zu Device.id
+topic_pattern   TEXT NOT NULL
+permission      TEXT NOT NULL                   -- 'read', 'write', 'readwrite'
+created_at      TEXT DEFAULT (datetime('now'))
+```
+
+**MQTT Sync Status Tabelle:**
+```sql
+id                  INTEGER PRIMARY KEY CHECK (id = 1)  -- Singleton
+pending_changes     INTEGER DEFAULT 0
+last_sync_at        TEXT
+last_sync_status    TEXT
+created_at          TEXT DEFAULT (datetime('now'))
+updated_at          TEXT DEFAULT (datetime('now'))
+```
+
 **Indexes:**
 - `idx_user_username` ON User(username)
+- `idx_user_parent` ON User(parent_user_id)
 - `idx_device_owner` ON Device(ownerId)
 - `idx_device_active` ON Device(isActive)
+- `idx_mqtt_credentials_device` ON mqtt_credentials(device_id)
+- `idx_mqtt_credentials_username` ON mqtt_credentials(mqtt_username)
+- `idx_mqtt_acl_device` ON mqtt_acl_rules(device_id)
 
 ---
 
@@ -360,8 +395,11 @@ graph LR
 
 ```mermaid
 erDiagram
+    USER ||--o{ USER : "parent_user_id"
     USER ||--o{ DEVICE : owns
     DEVICE ||--o{ LOCATION : tracks
+    DEVICE ||--o| MQTT_CREDENTIALS : has
+    DEVICE ||--o{ MQTT_ACL_RULES : has
 
     USER {
         string id PK
@@ -369,6 +407,7 @@ erDiagram
         string email
         string passwordHash
         string role
+        string parent_user_id FK
         datetime createdAt
         datetime lastLoginAt
     }
@@ -383,6 +422,33 @@ erDiagram
         datetime createdAt
     }
 
+    MQTT_CREDENTIALS {
+        int id PK
+        string device_id FK_UK
+        string mqtt_username UK
+        string mqtt_password_hash
+        boolean enabled
+        datetime created_at
+        datetime updated_at
+    }
+
+    MQTT_ACL_RULES {
+        int id PK
+        string device_id FK
+        string topic_pattern
+        string permission
+        datetime created_at
+    }
+
+    MQTT_SYNC_STATUS {
+        int id PK
+        int pending_changes
+        datetime last_sync_at
+        string last_sync_status
+        datetime created_at
+        datetime updated_at
+    }
+
     LOCATION {
         int id PK
         float latitude