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/services/db.py

@@ -557,6 +557,16 @@ async def list_cases(status: str | None = None, limit: int = 50) -> list[dict]:
     return [_row_to_case(r) for r in rows]
 
 
+async def delete_case(case_id: UUID) -> bool:
+    """Delete a case. Dependent rows in documents/document_chunks/qa_results
+    cascade automatically (schema-level ON DELETE CASCADE); audit_log rows
+    nullify their case_id reference. Returns True if a row was deleted."""
+    pool = await get_pool()
+    async with pool.acquire() as conn:
+        result = await conn.execute("DELETE FROM cases WHERE id = $1", case_id)
+    return result.endswith(" 1")
+
+
 async def update_case(case_id: UUID, **fields) -> dict | None:
     if not fields:
         return await get_case(case_id)