feat(integrations): add SerpAPI integration + generic handler

[lev-goryachev]

Summary

• Adds generic handler to `ckit_integrations_db.py`: any `fi_{name}.py` following the `Integration*` / `PROVIDER_NAME` / `called_by_model` pattern loads automatically without an explicit `elif` block
• Adds `fi_serpapi.py` — full implementation of SerpAPI, 96 method_ids covering all documented engines

Coverage: 96 method_ids

91 search engines: Amazon, Apple App Store, Baidu, Bing (search/news/images/maps/shopping/videos/copilot/reverse image), DuckDuckGo (search/news/images/maps/search assist), eBay, Facebook Profile, Google (search/scholar/news/images/maps/shopping/jobs/trends/videos/finance/flights/hotels/patents/play/lens/events/local/autocomplete/AI mode/AI overview/...), Home Depot, Naver, OpenTable, TripAdvisor, Walmart, Yahoo (search/images/shopping/videos), Yandex (search/images/videos), Yelp, YouTube

5 utility methods:

• `serpapi.account.get.v1` — account info & credits
• `serpapi.locations.search.v1` — available targeting locations
• `serpapi.search_archive.get.v1` — retrieve cached search by ID
• `serpapi.pixel_position.search.v1` — search with pixel positions
• `serpapi.pixel_position.archive.v1` — archived search with pixel positions

Changes

• `ckit_integrations_db.py`: add `import importlib/inspect`, replace `raise ValueError("Unknown integration")` with generic handler, relax `main_loop_integrations_init` signature
• `integrations/fi_serpapi.py`: new file, 346 lines

Test plan

• Verify `serpapi` loads when listed in bot integration allowlist
• `op=list_methods` returns all 96 method_ids
• `op=call` with `method_id=serpapi.search.google.v1` and `args.q=test` returns results (requires API key in `external_auth.serpapi`)