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/tests/test_download_errors.py

@@ -1,4 +1,4 @@
-"""Category classification of raw yt-dlp / ffmpeg download-failure strings (S2)."""
+"""Category classification of raw yt-dlp / ffmpeg download-failure strings."""
 
 from app.downloads import adapters
 from app.downloads import errors as dlerrors
@@ -71,7 +71,7 @@ def test_the_redacted_network_text_classifies_back_to_network():
     assert classify(dlerrors.NETWORK_REDACTED_MESSAGE) == "network"
 
 
-# Verbatim yt-dlp 2026.07.04 output from prod jobs, measured on dev 2026-09-13 (E11.5).
+# Verbatim yt-dlp 2026.07.04 output from prod jobs, measured on dev 2026-09-13.
 _FB_PARSE = (
     "ERROR: [facebook] 1736360027626346: Cannot parse data; please report this issue on  "
     "https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template."
@@ -196,7 +196,7 @@ def test_our_post_not_shown_verdict_is_the_refused_cookie():
 
 
 def test_our_post_without_a_video_verdict_names_the_group_link():
-    # The adapter's own text when the post page holds no single video (S4): the message and its rule and
+    # The adapter's own text when the post page holds no single video: the message and its rule and
     # catalog entry are pinned together, or an edit to the wording orphans the copy.
     msg = adapters.POST_WITHOUT_VIDEO_MESSAGE
     assert classify(msg) == "format_unavailable"