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/server.py

@@ -216,6 +216,22 @@ async def precedent_library_delete(case_law_id: str) -> str:
     return await plib.precedent_library_delete(case_law_id)
 
 
+@mcp.tool()
+async def precedent_link_cases(
+    case_law_id_a: str,
+    case_law_id_b: str,
+    relation_type: str = "same_case_chain",
+) -> str:
+    """קישור שתי פסיקות כקשורות (דו-כיווני, idempotent). relation_type: same_case_chain | overruled_by | distinguished."""
+    return await plib.precedent_link_cases(case_law_id_a, case_law_id_b, relation_type)
+
+
+@mcp.tool()
+async def precedent_unlink_cases(case_law_id_a: str, case_law_id_b: str) -> str:
+    """הסרת קישור בין שתי פסיקות (דו-כיווני)."""
+    return await plib.precedent_unlink_cases(case_law_id_a, case_law_id_b)
+
+
 @mcp.tool()
 async def precedent_extract_halachot(case_law_id: str) -> str:
     """הרצה מחדש של חילוץ הלכות לפסיקה קיימת. ההלכות הקיימות נמחקות, החדשות חוזרות לסטטוס pending_review."""