Add endpoint probe, rename cost_probe, and add PR/probe interval throttling

[BillJr99]

Summary

• Rename probe → cost_probe everywhere: source file (scripts/sources/cost_probe.py), class (CostProbeSource), config key (free_tier.cost_probe), state file (cost_probe_state.json), CLI flags (--cost-probe, --cost-probe-max). Back-compat migration copies old probe_state.json on first run.
• New endpoint_probe source (scripts/sources/endpoint_probe.py): actively calls each configured provider's GET /v1/models to discover new :free-suffix models and flag disappeared ones — gated by existing sync_on_startup / update_on_startup flags, throttled by free_tier.endpoint_probe.frequency_minutes (default 30).
• Interval checks on startup and per-request: _maybe_fire_interval_probes() (60 s in-memory gate) fires endpoint probe, cost probe, and PR creation as daemon threads when their respective intervals elapse.
• PR creation throttle: providers_pr.frequency_days config key; state tracked in pr_state.json next to the user config.
• 429 retry logic in DocsScraperBase.fetch(): up to 3 retries with Retry-After support, returns [] on exhaustion. Adds two new tests for the retry path.
• Race-condition fix in _build_models_list: startup warmup (only_if_empty=True) no longer overwrites _models_list_cache when a concurrent request or a cross-test daemon thread already populated it.

New config keys

"providers_pr": {
  "enabled": true,
  "frequency_days": 7
},
"free_tier": {
  "sync_on_startup": true,
  "update_on_startup": false,
  "endpoint_probe": {
    "frequency_minutes": 30,
    "timeout_sec": 10
  },
  "cost_probe": {
    "enabled": false,
    "autoremove": false,
    "frequency_days": 0
  }
}

State files (auto-created next to config.json):

• endpoint_probe_state.json — tracks last_probe_at for the endpoint probe
• cost_probe_state.json — tracks last_probe_at for the cost probe (migrated from probe_state.json)
• pr_state.json — tracks last_pr_at for PR throttling

Test plan

• All existing tests pass (no regressions)
• test_probe_source.py uses CostProbeSource from scripts.sources.cost_probe
• test_probe_throttle.py uses cost_probe_state.json path helpers
• test_docs_huggingface.py covers 429 retry and exhaustion paths
• test_real_model_name_unchanged no longer flaky (startup daemon thread race fixed)
• test_admin_api.py::test_put_maintenance_sets_flags checks free_tier.cost_probe key

🤖 Generated with Claude Code

https://claude.ai/code/session_01RkRhX4Znv73oPcZy41bnyS

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