feat: add SerpBase (Google) search provider via REST API

[gefsikatsinelou]

Summary

Add SerpBase as a Google search provider — returns structured Google search results via REST API with zero scraping maintenance.

Background

ApeRAG currently supports DuckDuckGo, Jina, and LLM.txt search providers. Google search is a commonly requested backend for research-quality results, but scraping Google directly is brittle and requires constant maintenance. SerpBase provides a reliable, structured Google Search API that fits naturally into the existing provider architecture.

Changes

• New: aperag/websearch/search/providers/serpbase_search_provider.py — REST API-based Google search provider (181 lines)
• Updated: aperag/websearch/search/providers/__init__.py — added SerpBaseSearchProvider import and export
• Updated: aperag/websearch/search/search_service.py — registered as "google" and "serpbase" in provider_registry

Design decisions

Decision	Rationale
API key via SERPBASE_API_KEY env var	Follows existing convention (Jina uses api_key in config / env pattern)
Graceful skip when key missing	Returns [] with a warning log — matches the "don't crash on missing config" philosophy
Uses aiohttp	Already a project dependency (used by Jina provider); no new packages
Mirrors existing provider structure	Same validation flow, domain filtering, locale mapping as DuckDuckGo/Jina providers

Testing

• When SERPBASE_API_KEY is set: performs Google search and returns structured results
• When key is unset: logs warning, returns [], other providers continue unaffected
• Follows existing provider interface exactly — drop-in compatible

Related

• SerpBase offers 100 free searches at https://serpbase.dev — no credit card required