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/migrations/069_obligations_vendor_link.sql

@@ -0,0 +1,3 @@
+-- Obligations: Vendor-Verknuepfung fuer Art. 28 DSGVO
+ALTER TABLE compliance_obligations
+  ADD COLUMN IF NOT EXISTS linked_vendor_ids JSONB DEFAULT '[]'::jsonb;

backend-compliance/migrations/070_loeschfristen_vendor_link.sql

@@ -0,0 +1,3 @@
+-- Loeschfristen: Vendor-Verknuepfung
+ALTER TABLE compliance_loeschfristen
+  ADD COLUMN IF NOT EXISTS linked_vendor_ids JSONB DEFAULT '[]'::jsonb;