feat(core): playlist per-item quality + incremental sync

ytdlp.rs:
- Add PlaylistItemProbe / PlaylistProbeResult (pub, serde::Serialize)
- Add private available_video_heights_from_value() helper for Value entries
- Add probe_playlist_qualities(): yt-dlp -J (full metadata, no flat flag)
  returns per-video quality lists in one subprocess call

capture.rs:
- Add per_item_quality: HashMap<String,String> and sync: bool to CaptureConfig
  (both Default; keyed by yt-dlp video ID, not URL)
- Add pub locator_to_playlist_url(): validates only the three playlist sources,
  expands shorthands; keeps locator_to_ytdlp_url's no-playlists contract
- Playlist capture block: sync-aware container resolution
  - sync + existing container → reuse it via complete_archive_run_item,
    skip already-archived children (by canonical URL) before creating
    run items so refresh_run_counters only counts new items
  - sync + no container → create normally (first sync run)
  - non-sync → always create fresh container (existing behaviour)
- Per-item quality: config.per_item_quality.get(id) falls back to child_quality

archive.rs:
- Add get_archived_playlist_child_urls(): returns HashSet of canonical URLs
  of all children under any container matching the playlist canonical URL
- Add find_container_entry_id_by_canonical_url(): returns most-recent
  container entry id (parent_entry_id IS NULL) for a given canonical URL

routes.rs: stub per_item_quality/sync on both CaptureConfig sites (server
agent will wire body fields in Phase 2)

crates/archivr-server/src/routes.rs

@@ -879,6 +879,8 @@ async fn capture_handler(
         modal_closer_enabled: Some(effective_modal_closer),
         reader_mode: body.reader_mode.unwrap_or(false),
         via_freedium: body.via_freedium.unwrap_or(true),
+        per_item_quality: std::collections::HashMap::new(),
+        sync: false,
     };
 
     // Spawn background capture.
@@ -998,6 +1000,8 @@ async fn rearchive_handler(
         modal_closer_enabled: None,
         reader_mode: false,
         via_freedium: false,
+        per_item_quality: std::collections::HashMap::new(),
+        sync: false,
     };
 
     let job_uid_bg = job_uid.clone();