refactor(plex): the item progress body extends the shared checkpoint

ItemProgressIn repeated ProgressIn's config, fields and bounds; it now adds only `final`. The
threshold comment points where the constants live now, and plexProgressBeacon's comment is back
on it.

backend/app/routes/plex/playback.py

@@ -15,7 +15,7 @@ from fastapi import (
     Response,
 )
 from fastapi.responses import FileResponse
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel
 from sqlalchemy import select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
@@ -23,7 +23,6 @@ from sqlalchemy.orm import Session
 from app.auth import current_user, resolved_user_id
 from app.db import SessionLocal, get_db
 from app.models import (
-    POSITION_MAX_S,
     PlexItem,
     PlexSeason,
     PlexShow,
@@ -39,7 +38,7 @@ from app.plex import watch_sync as plex_watch
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.routes.plex._shared import item_or_404
 from app.refusal import refused
-from app.routes._common import not_found
+from app.routes._common import ProgressIn, not_found
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
@@ -160,11 +159,11 @@ def subtitle(
     return Response(subtitles.shift_vtt(hit, offset), media_type="text/vtt")
 
 
-# Resume/watched thresholds. _PROGRESS_MIN_S matches the feed's PROGRESS_MIN_SECONDS (5), but
-# _FINISH_MARGIN_S is DELIBERATELY larger than the feed's FINISH_MARGIN_SECONDS (30 vs 10): this is
-# long-form content (movies/episodes with credits) and crossing it MARKS THE ITEM WATCHED, whereas
-# the feed's margin only clears a short video's resume bar — different domains, intentional divergence,
-# not drift. Keep them separate on purpose (see routes/feed.py).
+# Resume/watched thresholds. _PROGRESS_MIN_S matches PROGRESS_MIN_SECONDS (5), but _FINISH_MARGIN_S is
+# DELIBERATELY larger than FINISH_MARGIN_SECONDS (30 vs 10): this is long-form content
+# (movies/episodes with credits) and crossing it MARKS THE ITEM WATCHED, whereas the feed's and the
+# library's margin only clears a resume bar — different domains, intentional divergence, not drift.
+# Keep them separate on purpose (see routes/_common.py).
 _PROGRESS_MIN_S = 5
 
 _FINISH_MARGIN_S = 30
@@ -337,16 +336,9 @@ def _mark_finished(db: Session, user_id: int, item_id: int, now: datetime) -> bo
     return False
 
 
-class ItemProgressIn(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)
+class ItemProgressIn(ProgressIn):
+    """The shared checkpoint (its bounds included), plus whether the player is leaving the item."""
 
-    # float, truncated in the handler — the player may post a raw currentTime (a float); a plain
-    # `int` field would 422 it on the periodic progress checkpoint (the old `int(...)` truncated).
-    # 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)
     final: bool = False