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>

src/core_config_keys.py

@@ -29,6 +29,7 @@ CORE_CONFIG_KEYS = frozenset({
     'display',
     'sync',
     'plugin_system',
+    'fetch_service',
     # Older or optional core sections still found in existing config files.
     'logging',
     'network',