feat(syndesmos): external API integration (P3-06)

[forkwright]

Summary

• Adds the syndesmos crate: external API integration for Plex, Last.fm, and Tidal
• Implements ExternalIntegration trait with all four methods: notify_plex_import, scrobble, sync_tidal_want_list, get_artist_data
• Aggelia event handler for PlexNotifyRequired → Plex library refresh and ScrobbleRequired → Last.fm scrobble
• Per-service circuit breakers (Plex, Last.fm, Tidal) with configurable threshold (default: 5 failures) and cooldown (default: 5 min)
• Exponential-backoff retry: 1 initial + 3 retries at 2 s, 8 s, 32 s
• All integrations degrade gracefully when unconfigured (Ok(()) / Ok(None) / Ok(vec![]))
• SyndesmosConfig added to horismos (PlexConfig, LastfmConfig, TidalConfig, all Option<_>)
• Removes unused MockPlexApi::with_failures and fail_count — dead code containing a latent panic (unwrap_err() on a URL that reqwest accepts as valid)

Test count

36 tests, all green:

• retry: 7 tests (backoff, circuit breaker trip/reset, attempt count)
• events: 3 tests (PlexNotifyRequired, ScrobbleRequired, cancellation)
• plex/notify: 4 tests (section mapping, no-section graceful, by-section direct)
• lastfm/artist: 3 tests (found, not found, JSON parse)
• lastfm/scrobble: 2 tests (mock parameters, circuit path)
• lastfm/auth: 3 tests (auth URL, signature sort, format exclusion)
• tidal/wantlist: 5 tests (delta, event emit, no event, empty, parse)
• lib (SyndesmosService): 9 tests (all four methods: unconfigured + configured)

Validation results

cargo fmt --all -- --check     OK
cargo clippy -p syndesmos -- -D warnings   OK (zero warnings)
cargo test -p syndesmos        36 passed, 0 failed

Observations

• MockPlexApi::with_failures used reqwest::Client::new().get("http://invalid.test/").build().unwrap_err() to generate a reqwest::Error. Because http://invalid.test/ is a syntactically valid URL, build() succeeds and unwrap_err() would panic if this path were ever hit. The entire failure-simulation path was removed; retry behavior is tested generically in retry.rs.
• #[tokio::test(start_paused = true)] requires the test-util tokio feature. The prompt spec omitted it; correctly added to dev-dependencies.

[forkwright]

Duplicate — syndesmos already merged as #83