feat: Mini-Apps registry, Stammdaten backend persistence, bank accounts

- Register 6 mini-apps in kommunikation plugin (contact_picker, file_share,
  calendar_invite, mail_forward, ai_search, task_create)
- Connect AdressenTab to backend API (GET/POST/PATCH/DELETE /addresses)
- Create BankAccount model, schema, service, routes + migration 0033
- Connect KontenTab to backend API (GET/POST/PATCH/DELETE /bank-accounts)
- AI tools already working: 7 tools registered (hybrid_search, get_contact_mails, etc.)

app/models/bank_account.py

@@ -0,0 +1,30 @@
+"""BankAccount model — multiple bank accounts per tenant."""
+
+from __future__ import annotations
+
+import uuid
+
+from sqlalchemy import Boolean, String
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class BankAccount(Base, TenantMixin):
+    """Bank account entity — multiple accounts per tenant.
+
+    is_default: one default bank account per tenant.
+    """
+
+    __tablename__ = "bank_accounts"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    bank_name: Mapped[str] = mapped_column(String(100), nullable=False)
+    iban: Mapped[str] = mapped_column(String(34), nullable=False)
+    bic: Mapped[str | None] = mapped_column(String(11), nullable=True)
+    account_holder: Mapped[str | None] = mapped_column(String(200), nullable=True)
+    default_tax: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)