Add some types to coverstore

[cdrini]

Small refactor while trying to grok some of the coverstore code in response to coverstore investigation during high traffic event.

Technical

Testing

Screenshot

Stakeholders

[Copilot]

Pull request overview

This PR adds type annotations to several functions in the coverstore codebase to improve code clarity and type safety. The changes introduce type hints for function parameters and return types, and define two TypedDict-style classes (CoverDbDetails and PartialCoverDetails) to provide structure for cover detail objects.

• Added type annotations to utility functions (ol_things, ol_get) and core cover handling functions
• Introduced CoverDbDetails and PartialCoverDetails classes to type cover detail objects
• Refactored variable naming in the cover.GET method for better clarity (e.g., value → cover_id)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
openlibrary/coverstore/utils.py	Added type hints to ol_things and ol_get functions
openlibrary/coverstore/db.py	Introduced CoverDbDetails class and added type annotation to details function
openlibrary/coverstore/coverlib.py	Added TYPE_CHECKING import and type hint to read_image function
openlibrary/coverstore/code.py	Added PartialCoverDetails class, type annotations to multiple functions, and refactored variable naming in cover.GET

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[RayBB]

I didn't test this locally but we need to fix the key string issue.
Otherwise the types themselves look right.

Also today I learned about the _ support for large numbers.

[RayBB]

Key isn't a literal string of "key" it's dynamic

[RayBB]

This is a web storage because that's what the db returns.