fix(metadata): match ani-cli API requests

[MiguelRegueiro]

Summary

• Match ani-cli’s current AllAnime API request shape for metadata/search lookups.
• Use POST JSON with ani-cli-style headers instead of GET query params.
• Fall back to stored (N episodes) totals when metadata has no usable episode list.

Closes #13.

Testing

• cargo fmt --all -- --check
• cargo test --locked
• cargo clippy --all-targets --all-features -- -D warnings
• cargo audit --deny unsound
• Manual TUI smoke: metadata warnings no longer appear for the affected tracked entries.