feat: Datenbank-Schema & ER-Diagramm

📊 Schema (schema.sql):
- organizations (Multi-Tenancy)
- users (mit Rollen)
- contacts (mit DSGVO Felder)
- companies
- pipelines (JSONB stages)
- deals
- activities
- audit_logs (DSGVO)
- refresh_tokens

📈 ER-Diagramm (Mermaid)
🌱 Seed-Daten für Testing

Features:
- UUID Primary Keys
- Soft Delete (deleted_at)
- Auto-updated timestamps
- GIN Index für Tags
- Row-Level Security ready

docs/ER-DIAGRAM.md

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+# Pulse CRM - Entity Relationship Diagram
+
+```mermaid
+erDiagram
+    ORGANIZATIONS ||--o{ USERS : has
+    ORGANIZATIONS ||--o{ CONTACTS : has
+    ORGANIZATIONS ||--o{ COMPANIES : has
+    ORGANIZATIONS ||--o{ PIPELINES : has
+    ORGANIZATIONS ||--o{ DEALS : has
+    ORGANIZATIONS ||--o{ ACTIVITIES : has
+    ORGANIZATIONS ||--o{ AUDIT_LOGS : has
+
+    USERS ||--o{ CONTACTS : owns
+    USERS ||--o{ COMPANIES : owns
+    USERS ||--o{ DEALS : owns
+    USERS ||--o{ ACTIVITIES : creates
+    USERS ||--o{ REFRESH_TOKENS : has
+
+    COMPANIES ||--o{ CONTACTS : employs
+    COMPANIES ||--o{ DEALS : has
+
+    CONTACTS ||--o{ DEALS : associated
+    CONTACTS ||--o{ ACTIVITIES : has
+
+    PIPELINES ||--o{ DEALS : contains
+
+    DEALS ||--o{ ACTIVITIES : has
+
+    ORGANIZATIONS {
+        uuid id PK
+        string name
+        string slug UK
+        jsonb settings
+        string subscription_plan
+        timestamp created_at
+    }
+
+    USERS {
+        uuid id PK
+        uuid org_id FK
+        string email UK
+        string password_hash
+        string first_name
+        string last_name
+        string role
+        boolean is_active
+        timestamp last_login_at
+    }
+
+    CONTACTS {
+        uuid id PK
+        uuid org_id FK
+        uuid company_id FK
+        uuid owner_id FK
+        string first_name
+        string last_name
+        string email
+        string phone
+        string status
+        boolean gdpr_consent
+        timestamp deleted_at
+    }
+
+    COMPANIES {
+        uuid id PK
+        uuid org_id FK
+        uuid owner_id FK
+        string name
+        string industry
+        string website
+        string phone
+        timestamp deleted_at
+    }
+
+    PIPELINES {
+        uuid id PK
+        uuid org_id FK
+        string name
+        boolean is_default
+        jsonb stages
+    }
+
+    DEALS {
+        uuid id PK
+        uuid org_id FK
+        uuid pipeline_id FK
+        uuid contact_id FK
+        uuid company_id FK
+        uuid owner_id FK
+        string title
+        decimal value
+        string stage_id
+        string status
+        date expected_close_date
+    }
+
+    ACTIVITIES {
+        uuid id PK
+        uuid org_id FK
+        uuid user_id FK
+        uuid contact_id FK
+        uuid deal_id FK
+        string type
+        string subject
+        text description
+        timestamp due_date
+        boolean is_completed
+    }
+
+    AUDIT_LOGS {
+        uuid id PK
+        uuid org_id FK
+        uuid user_id FK
+        string action
+        string entity_type
+        uuid entity_id
+        jsonb changes
+        inet ip_address
+    }
+
+    REFRESH_TOKENS {
+        uuid id PK
+        uuid user_id FK
+        string token_hash
+        timestamp expires_at
+    }
+```
+
+## Tabellen-Übersicht
+
+| Tabelle | Beschreibung | Relationships |
+|---------|--------------|---------------|
+| `organizations` | Mandanten (Multi-Tenancy) | Parent für alle Daten |
+| `users` | Benutzer/Team-Mitglieder | Gehört zu Organization |
+| `contacts` | Personen/Leads | Gehört zu Organization, optional zu Company |
+| `companies` | Firmen | Gehört zu Organization |
+| `pipelines` | Sales Pipelines | Gehört zu Organization |
+| `deals` | Opportunities | Gehört zu Pipeline, Contact, Company |
+| `activities` | Notizen, Calls, Tasks | Gehört zu Contact oder Deal |
+| `audit_logs` | DSGVO Audit Trail | Protokolliert alle Änderungen |
+| `refresh_tokens` | JWT Refresh Tokens | Gehört zu User |
+
+## Indizes
+
+Alle Tabellen haben Indizes auf:
+- `org_id` - Multi-Tenancy Queries
+- Foreign Keys
+- Häufig gefilterte Felder (status, email, etc.)
+- `deleted_at` - Soft Delete Queries