Add Audible metadata provider via Audimeta

[cadric]

Closes #515

Summary

This adds a new audible metadata provider backed by the Audimeta API.

The provider supports:

• Audible/Audimeta metadata lookup without authentication
• region selection (us, ca, uk, au, fr, de, jp, it, in, es, br)
• ASIN book lookup
• ISBN lookup with fallback search
• series suggestions and series-order browsing
• richer audiobook metadata such as narrators, runtime, rating, subtitle, cover, publisher, and series info
• configurable Audimeta base URL, timeout, cache usage, default sort, and unreleased filtering

Notes

A few Audimeta-specific integration details were needed:

• send a meaningful User-Agent, otherwise Audimeta rejects requests with 403
• send the cache parameter in the format Audimeta expects
• use keywords for general search instead of query, which gave poor/irrelevant results for title-style
  searches

Validation

Tested locally with:

• python -m py_compile shelfmark/metadata_providers/audible.py
• python -m pytest tests/metadata/test_audible.py -v
• python -m pytest tests/metadata/test_metadata_provider_capabilities.py -v

Also verified manually in a Podman test container:

• searching for Discount Dan returns Audible title B0DXLXRNGG
• book details and series metadata load correctly

Scope

This PR intentionally keeps the change localized to the provider layer and docs:

• new Audible provider
• provider registration
• provider docs
• generated environment variable docs

[alexhb1]

Looks great! thanks for the work. I'll have a full look at this at the weekend.

One thing, you mentioned running test_audible.py. Is this safe to include in the PR also?

[cadric]

Looks great! thanks for the work. I'll have a full look at this at the weekend.

One thing, you mentioned running test_audible.py. Is this safe to include in the PR also?

I have added the test_audible.py file.

[cadric]

I pushed two small follow-up fixes:

1. Fixed Audible "View series" browsing so the frontend's id:<seriesId> format resolves correctly.
2. Cleaned Audible descriptions so HTML tags like <p> do not appear in the details modal.

Also added test coverage for both cases.