feat: link related precedents across court instances (SCHEMA_V11)

Add ability to mark case_law records as related (e.g. same appeal
through ועדת ערר → מנהלי → עליון):
- DB: case_law_relations join table (bidirectional, V11 migration)
- DB CRUD: add/remove/get_case_law_relations
- Service: get_precedent() now returns related_cases[]
- MCP: precedent_link_cases + precedent_unlink_cases tools
- REST: POST/DELETE /api/precedent-library/{id}/relations
- UI: RelatedCasesSection on detail page with search dialog and unlink

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

mcp-server/src/legal_mcp/services/precedent_library.py

@@ -438,13 +438,14 @@ async def delete_precedent(case_law_id: UUID | str) -> bool:
 
 
 async def get_precedent(case_law_id: UUID | str) -> dict | None:
-    """Get a precedent with its halachot attached."""
+    """Get a precedent with its halachot and related cases attached."""
     if isinstance(case_law_id, str):
         case_law_id = UUID(case_law_id)
     record = await db.get_case_law(case_law_id)
     if not record:
         return None
     record["halachot"] = await db.list_halachot(case_law_id=case_law_id, limit=500)
+    record["related_cases"] = await db.get_case_law_relations(case_law_id)
     return record