refactor price api in prep for fastapi

[RayBB]

Closes #
Again, very simple refactor to make it easier to share code as we migrate to the fastapi version

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[RayBB]

AI review:

Review of PR #11980
What it does: Refactors the price API by extracting get_price_data() as a static method, returning the dict instead of JSON-encoding it in the method.
Risks:

1. None - looks clean. The refactor simply:
   • Extracts logic into a reusable static method
   • Returns raw dict instead of json.dumps() in the extracted method
   • The GET() method still handles the JSON encoding
2. Minor note: The original code used i.asin or normalize_isbn(i.isbn) but now uses asin or normalize_isbn(isbn) - this is a slightly cleaner pattern but functionally equivalent.
3. No breaking changes - callers of GET() will see the same JSON output. Anyone calling get_price_data() directly (if it existed before) wouldn't have since it didn't exist.
   Verdict: Safe to merge. Standard refactor.