Refactor: Switch Watchlist functionality from TMDB API v4 to v3

[RalfKit]

Summary

This PR migrates all watchlist-related functionality from TMDB API v4 to API v3.

Reason

TMDB API v4 does not provide straightforward support for removing items from the default watchlist without managing custom list IDs. The default watchlist itself does not expose a usable list ID, which makes item removal unnecessarily complex.

Additionally, API v3 provides simpler session-based authentication and direct watchlist management via the numeric account_id, which better fits the current application requirements.

Changes

• Replaced v4 watchlist endpoints with TMDB API v3 equivalents
• Implemented session creation via v3 authentication
• Integrated numeric account_id retrieval for watchlist operations
• Updated add/remove/check watchlist logic to use v3 endpoints
• Removed unused v4 watchlist-related logic

Impact

• Simplifies watchlist management (add, remove, query)
• Removes dependency on v4 list handling for default watchlist
• Aligns authentication and account handling with TMDB v3 requirements

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud