feat(sdk): vendor-compliance cross-module integration — VVT, obligations, TOM, loeschfristen

Integrate the vendor-compliance module with four DSGVO modules to eliminate
data silos and resolve the VVT processor tab's ephemeral state problem.

- Reposition vendor-compliance sidebar from seq 4200 to 2500 (after VVT)
- VVT: replace ephemeral ProcessorRecord state with Vendor-API fetch (read-only)
- Obligations: add linked_vendor_ids (JSONB) + compliance check #12 MISSING_VENDOR_LINK
- TOM: add vendor TOM-controls cross-reference table in overview tab
- Loeschfristen: add linked_vendor_ids (JSONB) + vendor picker + document section
- Migrations: 069_obligations_vendor_link.sql, 070_loeschfristen_vendor_link.sql
- Tests: 12 new backend tests (125 total pass)
- Docs: update obligations.md + vendors.md with cross-module integration

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

backend-compliance/compliance/api/loeschfristen_routes.py

@@ -56,6 +56,7 @@ class LoeschfristCreate(BaseModel):
     responsible_person: Optional[str] = None
     release_process: Optional[str] = None
     linked_vvt_activity_ids: Optional[List[Any]] = None
+    linked_vendor_ids: Optional[List[Any]] = None
     status: str = "DRAFT"
     last_review_date: Optional[datetime] = None
     next_review_date: Optional[datetime] = None
@@ -86,6 +87,7 @@ class LoeschfristUpdate(BaseModel):
     responsible_person: Optional[str] = None
     release_process: Optional[str] = None
     linked_vvt_activity_ids: Optional[List[Any]] = None
+    linked_vendor_ids: Optional[List[Any]] = None
     status: Optional[str] = None
     last_review_date: Optional[datetime] = None
     next_review_date: Optional[datetime] = None
@@ -100,7 +102,7 @@ class StatusUpdate(BaseModel):
 # JSONB fields that need CAST
 JSONB_FIELDS = {
     "affected_groups", "data_categories", "legal_holds",
-    "storage_locations", "linked_vvt_activity_ids", "tags"
+    "storage_locations", "linked_vvt_activity_ids", "linked_vendor_ids", "tags"
 }