Implement P1 local matcher/ledger and P2 strategy engine.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/strategy/group.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from .clock import group_date_ymd
+
+
+def next_group_id(existing_count: int, now=None) -> str:
+    ymd = group_date_ymd(now)
+    n = int(existing_count) + 1
+    return f"G-{ymd}-{n:02d}"