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/tests/test_request_models.py

@@ -7,7 +7,7 @@ from pydantic import ValidationError
 
 from app.routes.channels import AttachTagIn, ChannelUpdateIn
 from app.routes.downloads import AdminSetQuotaIn
-from app.routes.feed import VideoProgressIn
+from app.routes._common import ProgressIn
 from app.routes.setup import SetupAdminIn
 from app.routes.tags import TagUpdateIn
 
@@ -52,10 +52,10 @@ def test_admin_quota_rejects_non_numeric_limit():
 def test_progress_accepts_fractional_seconds():
     # The player may post a raw currentTime (a float). The field must accept it (truncated in the
     # handler) — a plain `int` field would 422 on the progress hot path.
-    assert VideoProgressIn(position_seconds=12.7).position_seconds == 12.7
-    assert int(VideoProgressIn(position_seconds=12.7).position_seconds) == 12
+    assert ProgressIn(position_seconds=12.7).position_seconds == 12.7
+    assert int(ProgressIn(position_seconds=12.7).position_seconds) == 12
     with pytest.raises(ValidationError):
-        VideoProgressIn(position_seconds="not-a-number")
+        ProgressIn(position_seconds="not-a-number")
 
 
 # --- PATCH models: model_fields_set separates "absent" from "explicit null". ---