chore(comments): drop epic, sprint and review-round ids from the code

The code cited its plan instead of saying why: `E28 review J4`, `(R6 S3b)`, `S1 justified…`,
`deep-review round 5`. That plan lives in the private ops repo, so for every reader of this one the
citation was noise, and a sentence whose subject was the plan step ("round 2 showed…") hid its
lesson behind an id. Each one is gone and the "why" kept: a bare citation is deleted, a sentence
carrying a lesson is reworded around it ("a later attempt showed…").

Measured (`scripts/audit_metrics.py`, product code): 652 lines in 270 files carried an epic code by
the definition the next commit holds (507 / 241 by the old, two-digit-only one) -> 0. Across all
code, tests, scripts and configs: ~1100 epic-code lines plus ~320 sprint/round ids.

Behaviour unchanged, checked mechanically: every changed .py file has the same AST once docstrings
are blanked; css/sh/toml/yml/json files are identical once comments are stripped; the only
non-comment changes in TS/JS are test titles, one description string in a contract table, and five
ESLint messages, whose text is all the gates snapshot diff holds. Not touched: Markdown (docs and
the changelog keep their history), Alembic migrations, TV episode notation (`S2 · E5`), DESIGN.md
ids (`C1`, `B4`, `S3h`), tracker ids (`SEC-16`, `DX-17`).

One stale claim fixed on the way: test_hls_reaper's docstring said the player checkpoints "every
10 s whether or not it is playing", which plex/stream.py calls wrong; it now says what the code does.

backend/app/routes/plex/playback.py

@@ -169,7 +169,7 @@ _PROGRESS_MIN_S = 5
 
 _FINISH_MARGIN_S = 30
 
-# S2 write-back throttle: the resume position is now pushed to Plex on the periodic (non-final)
+# Write-back throttle: the resume position is now pushed to Plex on the periodic (non-final)
 # checkpoint too — not only on the final pause/unmount flush — so Plex (the source of truth) stays
 # current in near-real-time. To keep a single watch from spraying `/:/timeline` calls, a push is
 # rate-limited to at most one per item per user every _PUSH_THROTTLE_S seconds; a `final` checkpoint
@@ -360,7 +360,7 @@ def item_progress(
 ) -> dict:
     """Checkpoint the resume position while playing (near the end → mark watched + clear it).
 
-    S2 write-back: the resume position is mirrored to Plex (the source of truth) on the periodic
+    Write-back: the resume position is mirrored to Plex (the source of truth) on the periodic
     checkpoint too, not only on the final flush — throttled to ≥_PUSH_THROTTLE_S per item (see
     `_should_push_resume`). `final` (sent on pause / seek / pagehide / unmount) pushes immediately and
     resets the throttle window, so Plex sees a settled position the instant the user pauses or seeks."""
@@ -696,7 +696,7 @@ def stream_hls_file(
     (viewer, rating_key) and a seek-restart replaces the viewer's registered one, while the DIRECTORY
     is per (key, viewer, offset, tag) — so resolving by the key alone handed the old player the new
     session's `seg_12.ts`, produced from a different offset under `-copyts`: wrong
-    bytes with a 200, which the player has no way to notice (deep-review round 5). A request for a
+    bytes with a 200, which the player has no way to notice. A request for a
     session that is no longer the registered one is a 404, and the player reclaims at the playhead.
 
     That reclaim is also why a request with no live session is no longer auto-started here: the
@@ -728,7 +728,7 @@ def stream_hls_file(
     # `proc` so a session whose ffmpeg has already exited answers at once instead of polling out the
     # whole timeout: this is a SYNC route on the ~40-slot threadpool, holding a pooled DB connection
     # with it, so every such wait is a worker (and a connection) parked for 20 s — an ordinary seek
-    # past the produced region was enough to do it (round 5).
+    # past the produced region was enough to do it.
     if not plex_stream.wait_for(path, timeout, proc=s.proc):
         raise refused(404, "not_ready")
     try: