feat: combat stats backend + frontend (Mag-Tools style)

Backend:
- db_async.py: new combat_stats + combat_stats_sessions tables
- main.py: combat_stats message handler with DB upsert (lifetime +
  session snapshots), in-memory live_combat_stats dict, broadcast
  to browser clients.
- REST: GET /combat-stats and GET /combat-stats/{character_name}

Frontend:
- index.html: new "Combat Stats" sidebar link
- script.js: full Combat Stats window with two panels:
  - Top: monster list (name, kills, dmg recv, dmg given) with
    clickable rows and "All" aggregate, matching CombatTrackerGUI.cs
  - Bottom: damage breakdown grid matching CombatTrackerGUIInfo.cs
    layout — element × attack type matrix (Mel/Msl + Magic columns),
    Attacks (hit%), Evades (%), Resists (%), A.Surges (%), C.Surges (%),
    normal Avg/Max, Crits (%), Crit Avg/Max, Total Damage.
  - Session / Lifetime toggle button
- style.css: combat-stats-toggle styles

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

db_async.py

@@ -83,6 +83,26 @@ rare_stats_sessions = Table(
     Column("session_id", String, primary_key=True),
     Column("session_rares", Integer, nullable=False, default=0),
 )
+# Per-character persistent combat stats (lifetime accumulation, Mag-Tools style)
+combat_stats = Table(
+    "combat_stats",
+    metadata,
+    Column("character_name", String, primary_key=True),
+    Column("timestamp", DateTime(timezone=True), nullable=False),
+    Column("stats_data", JSON, nullable=False),
+)
+
+# Per-session combat stats snapshots (session history)
+combat_stats_sessions = Table(
+    "combat_stats_sessions",
+    metadata,
+    Column("id", Integer, primary_key=True),
+    Column("character_name", String, nullable=False, index=True),
+    Column("session_id", String, nullable=False, index=True),
+    Column("timestamp", DateTime(timezone=True), nullable=False, index=True),
+    Column("stats_data", JSON, nullable=False),
+)
+
 # Table for recording spawn events (mob creates) for heatmap analysis
 spawn_events = Table(
     # Records individual mob spawn occurrences for heatmap and analysis