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/plugin_system/plugin_executor.py

@@ -10,6 +10,7 @@ from typing import Any, Dict, Optional, Callable
 from threading import Thread
 import logging
 
+from src.common.fetch_service import plugin_scope
 from src.exceptions import PluginError
 from src.logging_config import get_logger
 from src.error_aggregator import record_error
@@ -83,7 +84,10 @@ class PluginExecutor:
         
         def target():
             try:
-                result_container['value'] = operation()
+                # Fetches made by the operation (and by threads the core
+                # starts from it) are counted against this plugin.
+                with plugin_scope(plugin_id):
+                    result_container['value'] = operation()
                 result_container['completed'] = True
             except Exception as e:
                 result_container['exception'] = e