Add eBay comparable-based physical asset valuations

[Sumedh-Joshi]

Implements external comparable retrieval and explicit no-comps behavior for physical asset valuations, via eBay's Browse API.

• Query eBay Browse API for used/refurbished USD listings using the confirmed label, brand, and model (client-credentials OAuth2, token cached in memory for its ~2hr lifetime).
• Return usable USD prices, source, listing title, and condition as structured evidence; calculate the estimate from the median.
• Return a null value and low-confidence explanation when no usable comps are found.
• New: falls back to the existing LLM price guess (AiEstimatedValuationService) when eBay itself fails — missing credentials, network error, rate limit — instead of surfacing "Failed to retrieve comparable market listings." to the user. A genuine zero-results response (the call succeeded, no comps exist) is left as-is; that's not a failure.
• Add credential configuration (Ebay:ClientId/Ebay:ClientSecret) and tests for OAuth flow, token reuse, request construction, response mapping (including condition/currency filtering), empty results, error handling without leaking the client secret, disk-cache reuse, and valuation/fallback behavior.

Live-verified against eBay's real sandbox (not just mocks): confirmed the OAuth flow, that eBay's own conditions:{...} filter does not reliably exclude "New" listings (hence the client-side conditionId re-check), and that sandbox inventory is sparse for real product names — which is exactly why the fallback matters. Rebuilt the container with real sandbox credentials and clicked Revalue on a live item: real token fetch, real Browse API search, result cached to App_Data/ebay-search-cache/*.json.

Note: this branch previously regressed to a SerpApi Google-Search scrape (see commit history) after the original eBay integration shipped. That code is fully removed here; eBay is the only market-data provider again.

Closes #65
Closes #66
Closes #67