[Repo] Enhanced toggleWatchlist (Fetch + Cache)

[mukesh-5059]

Difficulty: Hard
Location: app/src/main/java/com/example/movies_db/data/repository/MovieRepositoryImpl.kt
Description:
When adding to watchlist from a search result (which might have partial data), we must:

1. Fetch the full movie details (runtime, tagline, complete overview) from API.
2. Fetch the providers.
3. Save everything to Room.
4. Pre-cache images (Issue [Repo] Enhanced toggleWatchlist (Fetch + Cache) #41).
   All this must happen atomically or with proper error handling.