feat(fetch): shared fetch service, stage 1 (pooling, merging, host budgets, counters) (#702)

Core's own HTTP fetch paths (APIHelper, fetch_espn_scoreboard and its date chunks, BackgroundDataService, BaseOddsManager.get_odds) go through one service in src/common/fetch_service.py: shared connection pools per retry policy, merged identical in-flight GETs, per-host token-bucket budgets (fetch_service.rate_limits), and per-plugin request counters published to GET /api/v3/plugins/fetch-stats. Return values, exceptions, cache keys, TTLs and retry policies are unchanged. Core-internal in this release; plugins should not import it directly yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/fixtures/api_v3_url_map.json

@@ -467,6 +467,15 @@
       "POST"
     ]
   ],
+  [
+    "/api/v3/plugins/fetch-stats",
+    "api_v3.get_fetch_stats",
+    [
+      "GET",
+      "HEAD",
+      "OPTIONS"
+    ]
+  ],
   [
     "/api/v3/plugins/health",
     "api_v3.get_plugin_health",