Add API cost tracking to admin dashboard

Track estimated Anthropic API costs per request across all Claude API
call sites (recommender, analyze, artist-dive, generate-playlist, crate,
rabbit-hole, playlist-fix, timeline, compatibility). Log token usage and
estimated cost to the app logger. Aggregate costs in admin stats endpoint
and display total/today costs and token usage in the admin dashboard.

backend/app/api/endpoints/admin.py

@@ -1,5 +1,6 @@
 import logging
 import io
+import re
 from datetime import datetime, timezone, timedelta
 
 from fastapi import APIRouter, Depends, HTTPException, Query
@@ -113,6 +114,28 @@ async def get_stats(
         for row in user_stats_result.all()
     ]
 
+    # Calculate API costs from log buffer
+    total_cost = 0.0
+    today_cost = 0.0
+    total_tokens_in = 0
+    total_tokens_out = 0
+    for log in _log_buffer:
+        if 'API_COST' in log.get('message', ''):
+            msg = log['message']
+            cost_match = re.search(r'cost=\$([0-9.]+)', msg)
+            if cost_match:
+                c = float(cost_match.group(1))
+                total_cost += c
+                # Check if today
+                if log['timestamp'][:10] == now.strftime('%Y-%m-%d'):
+                    today_cost += c
+            input_match = re.search(r'input=(\d+)', msg)
+            output_match = re.search(r'output=(\d+)', msg)
+            if input_match:
+                total_tokens_in += int(input_match.group(1))
+            if output_match:
+                total_tokens_out += int(output_match.group(1))
+
     return {
         "users": {
             "total": total_users,
@@ -131,6 +154,12 @@ async def get_stats(
             "saved": saved_recs,
             "disliked": disliked_recs,
         },
+        "api_costs": {
+            "total_estimated": round(total_cost, 4),
+            "today_estimated": round(today_cost, 4),
+            "total_input_tokens": total_tokens_in,
+            "total_output_tokens": total_tokens_out,
+        },
         "user_breakdown": user_breakdown,
     }