Fix content rating filter for titles without US classifications

[spekta-23]

Description of the bug

The global content rating filter can incorrectly hide valid movies and TV shows when the selected filter is restrictive, such as PG-13, and the title does not have a US classification. The issue is caused by the rating flow relying too heavily on US certifications and, in the TMDB catalog path, previously applying a US-only certification filter before metadata enrichment.

As a result, titles that have non-US/local classifications but no US classification can be filtered out even when their local classification is within the configured maximum rating.

Affected feature

Content Filters → Content Rating

The affected setting is the global ageRating option, which supports None, G, PG, PG-13, R, and NC-17. The project treats series ratings as TV equivalents, for example PG-13 maps to TV-14. 

Steps to reproduce

With ageRating=PG-13, some titles are incorrectly hidden:

• Tall Girl appears, but Tall Girl 2 is filtered out.
• Ben 10 Omniverse appears, but Pokemon is filtered out.

Expected behavior

The content rating filter should:

Fetch catalog/search/trending results.

Enrich each result with metadata, including all available rating information.

Evaluate the result against the user’s configured maximum rating.

Consider both canonical/US and local classifications when available.

Keep a title if at least one known, interpretable rating is at or below the configured maximum.

For example, a movie with no US rating but a local 12 classification should be allowed under a PG-13 filter. The documented intended flow now states that the filter checks app_extras.certification, app_extras.certificationLocal, certification, and contentRating, and normalizes non-US/numeric labels such as 12, 12+, 16, 18, MA15+, and R18+.

Environment

Self-hosted (Docker)

Server Logs / Browser Console Errors

[spekta-23]

With ageRating=PG-13, some titles are incorrectly hidden:

• Tall Girl appears, but Tall Girl 2 is filtered out.
• Ben 10 Omniverse appears, but Pokemon is filtered out.

The common pattern is missing or incomplete US classification data while local classification data exists.

Root cause

TMDB Discover certification filtering requires a single certification_country. Using US for the global rating filter removes titles that only have non-US/local ratings before metadata enrichment can attach certificationLocal.

Fix

• Stop injecting the global content rating setting into TMDB Discover as certification_country=US.
• Apply global content rating filtering after metadata enrichment.
• Add a shared rating utility that:
  • maps movie settings to TV equivalents,
  • normalizes US, TV, numeric, and common local ratings,
  • checks all candidate rating fields,
  • allows content if any known rating is within the configured maximum.
• Reuse the same helper in catalog and trending filters.
• Document the classification and filter flow.