Scope git history to current diagram by default

- History panel defaults to "This diagram" / "All" toggle
- "This diagram" filters git log to commits that touched only that
  diagram's JSON file (git log -- diagrams/0001_*.json)
- Automatically switches to "All" when no diagram is open
- Restore flow still works per-commit, per-file

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/git_mgr.py

@@ -185,9 +185,13 @@ def git_status():
 
 
 @router.get("/log")
-def git_log():
+def git_log(diagram_id: Optional[int] = None):
     repo_root = _repo_root()
-    r = _run("log", "--pretty=format:%H|%h|%s|%an|%ai", "-30", cwd=repo_root)
+    cmd = ["log", "--pretty=format:%H|%h|%s|%an|%ai", "--follow", "-50"]
+    if diagram_id is not None:
+        # Filter to commits that touched this diagram's file (glob by id prefix)
+        cmd += ["--", f"diagrams/{diagram_id:04d}_*.json"]
+    r = _run(*cmd, cwd=repo_root)
     if r.returncode != 0:
         return []
     entries = []