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.

Dockerfile

@@ -47,13 +47,13 @@ RUN adduser --disabled-password --gecos "" appuser \
     && mkdir -p /downloads \
     && chown appuser /app /downloads
 
-# ffmpeg: merge separate video+audio streams + postprocessing, and the Plex VAAPI transcode (E6 S1).
+# ffmpeg: merge separate video+audio streams + postprocessing, and the Plex VAAPI transcode.
 # intel-media-va-driver (iHD): the VA driver ffmpeg's h264_vaapi needs — libva2 ships in the base but
 # no driver, so without this `vainfo`/`h264_vaapi` fail and transcode silently falls back to libx264.
 # Harmless on hosts with no /dev/dri (the driver just goes unused). Deno: the JavaScript runtime yt-dlp
 # uses to solve YouTube's "n" signature challenge for the web player clients (required alongside the PO
 # token, else only storyboards are offered). yt-dlp auto-detects deno on PATH.
-# `apt-get upgrade` (E32): the base image is a snapshot, and the Debian security fixes released since it
+# `apt-get upgrade`: the base image is a snapshot, and the Debian security fixes released since it
 # was cut are only in the archive — the first Trivy scan found one (pcre2) fixed upstream but not in it.
 ARG DENO_VERSION=v2.9.1
 RUN apt-get update \
@@ -81,7 +81,7 @@ RUN printf 'Types: deb\nURIs: http://deb.debian.org/debian\nSuites: bookworm\nCo
 # Install from the PINNED lock (requirements.lock, generated from requirements.txt) so the image is
 # byte-reproducible and the exact versions are the ones the gate's pip-audit scanned — not whatever
 # floats up from the floor ranges on build day. Regenerate the lock when requirements.txt changes (see its header).
-# pip itself is removed once it has installed the lock (E32): nothing runs it at runtime, and it carries
+# pip itself is removed once it has installed the lock: nothing runs it at runtime, and it carries
 # its own VENDORED copies of urllib3, msgpack and setuptools, which Trivy reports as the image's own —
 # an attack surface and a CVE stream for a tool the running app never calls.
 COPY backend/requirements.lock .