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>

web/app.py

@@ -4380,6 +4380,37 @@ async def precedent_library_delete(case_law_id: str):
     return {"deleted": True, "case_law_id": case_law_id}
 
 
+class PrecedentRelationRequest(BaseModel):
+    related_id: str
+    relation_type: str = "same_case_chain"
+
+
+@app.post("/api/precedent-library/{case_law_id}/relations")
+async def precedent_add_relation(case_law_id: str, req: PrecedentRelationRequest):
+    try:
+        a = UUID(case_law_id)
+        b = UUID(req.related_id)
+    except ValueError:
+        raise HTTPException(400, "case_law_id לא תקין")
+    if not await db.get_case_law(a):
+        raise HTTPException(404, "פסיקה לא נמצאה")
+    if not await db.get_case_law(b):
+        raise HTTPException(404, f"פסיקה קשורה {req.related_id} לא נמצאה")
+    await db.add_case_law_relation(a, b, req.relation_type)
+    return {"linked": True, "case_law_id": case_law_id, "related_id": req.related_id}
+
+
+@app.delete("/api/precedent-library/{case_law_id}/relations/{related_id}")
+async def precedent_remove_relation(case_law_id: str, related_id: str):
+    try:
+        a = UUID(case_law_id)
+        b = UUID(related_id)
+    except ValueError:
+        raise HTTPException(400, "case_law_id לא תקין")
+    await db.remove_case_law_relation(a, b)
+    return {"unlinked": True, "case_law_id": case_law_id, "related_id": related_id}
+
+
 # Halacha and metadata extraction are LLM-driven and rely on the local
 # `claude` CLI via mcp-server/services/claude_session.py — they CANNOT run
 # from this container (no CLI, no claude.ai session). The endpoints below