Add case_delete: MCP tool + DELETE endpoint + DB helper

Wires a new case-deletion path across the three layers that needed it:

- db.delete_case(case_id) — single SQL DELETE; documents, chunks, and
  qa_results cascade via existing schema FKs, audit_log nullifies.
- cases_tools.case_delete(case_number, remove_files=False) — MCP tool
  wrapper. File tree on disk is kept by default (audit trail); pass
  remove_files=True for a hard delete.
- DELETE /api/cases?case_number=... — FastAPI endpoint taking the case
  number as a QUERY param rather than a path segment. Case numbers
  like "1000/0426" can't be passed through a path parameter because
  FastAPI routing decodes %2F before matching, so a query param is
  the only shape that works for historical data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/server.py

@@ -102,6 +102,12 @@ async def case_update(
     )
 
 
+@mcp.tool()
+async def case_delete(case_number: str, remove_files: bool = False) -> str:
+    """מחיקת תיק ערר. קבצים בדיסק נשארים אלא אם remove_files=true."""
+    return await cases.case_delete(case_number, remove_files)
+
+
 # Documents
 @mcp.tool()
 async def document_upload(