feat(player): the OS media controls, the basic keys and resume for the library and shared-link players

Plex, library and /watch tell the operating system what plays (title, line, picture) and take its
play / pause / skip / seek through one hook (lib/mediaSession); the Plex player drives it with its own
seek and clock, so the lock screen shows the film's position, not the stream's. The YouTube player is
left alone: its iframe already publishes all of it, and the OS routes to the frame that plays.

The library and /watch players get the basic keys (lib/mediaKeys, the keymap's mediaPlayer rows) and
open where the viewer stopped (lib/resume): the library keeps the point on the server, per user and
library item (library_states, POST /api/downloads/{id}/progress, listed back on both listings), the
shared link in this browser. A started library row continues, its menu can start it over, and its
picture carries the cards' progress bar. The resume rule (too early / near the end = from the start)
is one function shared with the YouTube route.

The library dialog opens focused on its panel (Modal initialFocus "player"): on the ✕, Space closed
the film. While the media element itself has the focus, Space and the arrows stay its own controls'
— the browser acts on them whatever a listener does, and one Space played and paused.

backend/app/routes/feed.py

@@ -1,7 +1,7 @@
 from datetime import date, datetime, timezone
 
 from fastapi import APIRouter, Depends, Query
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel
 from sqlalchemy import and_, func, or_, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
@@ -26,14 +26,19 @@ from app.feedquery import (
     windowed_relevance_rows,
 )
 from app.models import (
-    POSITION_MAX_S,
     ChannelTag,
     Tag,
     User,
     Video,
     VideoState,
 )
-from app.routes._common import get_or_404, youtube_refusal, not_found
+from app.routes._common import (
+    ProgressIn,
+    get_or_404,
+    kept_position,
+    not_found,
+    youtube_refusal,
+)
 from app.sync.videos import parse_iso8601_duration
 from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
@@ -44,14 +49,6 @@ router = APIRouter(prefix="/api", tags=["feed"])
 # "saved" used to be a status; it's now membership in the built-in Watch later playlist.
 VALID_STATES = {"new", "watched", "hidden"}
 
-# Resume-position thresholds (mirror the client): positions below this are "didn't
-# really start" and within this of the end are "basically finished" — neither is worth
-# storing, so we clear the position instead (the video shows no progress bar).
-# NB: the Plex player intentionally uses a LARGER finish margin (30s) because it marks
-# long-form items watched, not just clears a resume bar — see routes/plex/playback.py.
-PROGRESS_MIN_SECONDS = 5
-FINISH_MARGIN_SECONDS = 10
-
 
 # Shared query parameters for /feed and /feed/count.
 def _feed_params(
@@ -326,23 +323,10 @@ def clear_video_state(
     return {"ok": True, "video_id": video_id, "status": "new", "position_seconds": 0}
 
 
-class VideoProgressIn(BaseModel):
-    # Finite only: JSON parsing here accepts `Infinity`/`NaN`, and the handler's `int()` turned
-    # one into a 500.
-    model_config = ConfigDict(allow_inf_nan=False)
-
-    # Accept a float and truncate (below) — the player may post a raw currentTime (a float), which
-    # the old `int(payload.get(...))` silently truncated. A plain `int` field would 422 that on the
-    # 10s progress hot path, losing the resume position; `float` keeps the old tolerance.
-    # Bounded above by the int4 column (see POSITION_MAX_S): past it the flush was a 500.
-    position_seconds: float = Field(0, le=POSITION_MAX_S)
-    duration_seconds: float = Field(0, le=POSITION_MAX_S)
-
-
 @router.post("/videos/{video_id}/progress")
 def set_video_progress(
     video_id: str,
-    body: VideoProgressIn,
+    body: ProgressIn,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -350,22 +334,15 @@ def set_video_progress(
     Stores a per-user position on the video_states row without touching watch status, so a
     partially-watched video can render a progress bar and match the "in progress" filter.
     Trivially-early and near-finished positions clear the position rather than store it."""
-    position = int(body.position_seconds)
-    duration = int(body.duration_seconds)
-    if position < 0:
-        position = 0
+    position = kept_position(body)
 
     get_or_404(db, Video, video_id, what="video")
 
-    # Decide whether this position is worth keeping (else clear it).
-    near_end = duration > 0 and position > duration - FINISH_MARGIN_SECONDS
-    keep = position >= PROGRESS_MIN_SECONDS and not near_end
-
     row = db.execute(
         select(VideoState).where(VideoState.user_id == user.id, VideoState.video_id == video_id)
     ).scalar_one_or_none()
 
-    if not keep:
+    if not position:
         # Nothing meaningful to store: drop a status-less row entirely, otherwise just
         # zero the position (a saved/hidden video keeps its status).
         if row is not None: