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/tools/cases.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import shutil
 import subprocess
 from pathlib import Path
 from uuid import UUID
@@ -214,3 +215,37 @@ async def case_update(
         )
 
     return json.dumps(updated, default=str, ensure_ascii=False, indent=2)
+
+
+async def case_delete(case_number: str, remove_files: bool = False) -> str:
+    """מחיקת תיק ערר. מסיר את התיק מ-DB עם cascade לכל המסמכים והטענות.
+
+    Args:
+        case_number: מספר תיק הערר
+        remove_files: האם למחוק גם את תיקיית הדיסק (drafts, git repo).
+                      ברירת מחדל False — ה-DB נמחק אבל הקבצים נשמרים לגיבוי.
+    """
+    case = await db.get_case_by_number(case_number)
+    if not case:
+        return json.dumps(
+            {"deleted": False, "reason": f"תיק {case_number} לא נמצא."},
+            ensure_ascii=False,
+        )
+
+    case_id = UUID(case["id"])
+    ok = await db.delete_case(case_id)
+
+    result = {
+        "deleted": ok,
+        "case_number": case_number,
+        "case_id": str(case_id),
+        "removed_files": False,
+    }
+
+    if ok and remove_files:
+        case_dir = config.find_case_dir(case_number)
+        if case_dir.exists():
+            shutil.rmtree(case_dir, ignore_errors=True)
+            result["removed_files"] = True
+
+    return json.dumps(result, ensure_ascii=False, indent=2)