Add watch history system overview

docs/watch-history-system.md

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+# Watch history system overview
+
+This doc ties the playback telemetry in `js/services/watchHistoryTelemetry.js` to
+our watch history logging pipeline documented in
+[`docs/watch-history-logging.md`](watch-history-logging.md).
+
+## System map
+
+* **Playback telemetry** (`WatchHistoryTelemetry`) listens to the active `<video>`
+  element and decides when a watch should be logged.
+* **Watch history service** (`WatchHistoryService`) batches views into a local
+  queue, publishes snapshots to relays, and emits in-app events when the queue or
+  snapshots change.
+* **Watch history logging pipeline** (v2) describes how snapshots are encrypted
+  and published to relays. See `docs/watch-history-logging.md` for the wire
+  format and publish cadence.
+
+## What gets captured
+
+### Playback telemetry fields
+
+`WatchHistoryTelemetry` watches playback and only logs a view once the viewer
+passes the threshold duration (default 12 seconds). It also tracks a per-viewer
+cooldown key so the same actor doesn’t log the same pointer repeatedly in a
+single session. The telemetry payload stores a sanitized slice of video metadata
+(id, title, thumbnail, pubkey, created_at) and intentionally omits playback
+endpoints (URL/magnet/hash) so these never get persisted as watch history
+metadata.
+
+### Watch history snapshots
+
+Once a view is accepted, the telemetry layer forwards the pointer to the watch
+history service. The service packages pointer-only entries into watch-history
+snapshots (chunked NIP-04 encrypted events) and emits a pointer index event. The
+payload shape and relay publication sequence are documented in
+`docs/watch-history-logging.md`.
+
+## Where data is stored
+
+* **Local queue:** `WatchHistoryService` uses `localStorage` to persist the
+  in-flight view queue (`bitvid:watch-history:queue:v1`) so the history survives
+  refreshes even before a snapshot succeeds.
+* **Local cache:** A rolling watch-history cache (TTL controlled via
+  `WATCH_HISTORY_CACHE_TTL_MS`) keeps recently synced history available without
+  hitting relays on every page load.
+* **Relay snapshots:** Chunked, encrypted watch-history events are sent to relays
+  using the kind and list identifier configured in `config/instance-config.js`.
+
+> **Roadmap note:** Current watch history is **not** stored as an encrypted
+> local list. We plan to add encrypted local history in the future.
+
+## Privacy considerations
+
+* Watch history metadata is intentionally sanitized to exclude playback URLs,
+  magnets, and hashes; only presentation-safe fields are kept for history
+  rendering.
+* Relay payloads store pointer tags rather than full metadata, ensuring that
+  relays don’t learn titles or thumbnails unless an operator opts into local
+  metadata caching (see `docs/watch-history-logging.md`).
+* Session actors are treated as local-only history when they cannot publish to
+  relays; the history service retains those pointers locally without attempting
+  network sync.
+
+## Event emission and consumption
+
+### Telemetry → watch history service
+
+`WatchHistoryTelemetry.preparePlaybackLogging` attaches listeners to the video
+player. When the threshold is hit, it calls `watchHistoryService.publishView`
+(when available) or falls back to `nostrClient.recordVideoView`. Successful
+publish results are also forwarded to `ingestLocalViewEvent` so the view counter
+can update immediately.
+
+### Watch history service events
+
+`WatchHistoryService` emits events to subscribers and to the browser via the
+`bitvid:watchHistory` custom event. Any UI component can subscribe to these
+updates and refresh the watch-history view or related counters.
+
+## Configuration and sampling
+
+* **Telemetry sampling threshold:** `WatchHistoryTelemetry` defaults to 12
+  seconds before a view is logged. Integrations can override
+  `viewThresholdSeconds` when constructing the class.
+* **Feature flag:** Watch-history v2 is guarded by `FEATURE_WATCH_HISTORY_V2` in
+  `js/constants.js`.
+* **Instance configuration:** `config/instance-config.js` exposes watch-history
+  limits (max items, payload size), batching controls
+  (`WATCH_HISTORY_BATCH_RESOLVE`, `WATCH_HISTORY_BATCH_PAGE_SIZE`), and cache TTL
+  (`WATCH_HISTORY_CACHE_TTL_MS`) for operator tuning.
+
+## Related docs
+
+* [`docs/watch-history-logging.md`](watch-history-logging.md)
+* [`docs/instance-config.md`](instance-config.md)