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_failure_catalog.py

@@ -1,4 +1,4 @@
-"""DB-backed tests for the download failure catalog (E11.5 S1) at the places a failure is WRITTEN.
+"""DB-backed tests for the download failure catalog at the places a failure is WRITTEN.
 
 `test_download_errors` pins which entry a message matches; these pin that every writer hands the
 matcher what it needs — the link and whether the user's cookie went with it — and that a stored entry
@@ -8,7 +8,7 @@ never outlives the failure it describes:
 - Retry / Re-download, which requeue the job;
 - the `0084` backfill of the rows that failed before the catalog existed;
 - the downloads list, which is how the entry reaches the UI.
-It also holds the one success-path check of the same child: the duration it stores (E11.5 S3).
+It also holds the one success-path check of the same child: the duration it stores.
 """
 
 import shutil
@@ -226,7 +226,7 @@ def test_the_downloads_list_carries_the_catalog_entry(db, as_user):
 
 @pytest.mark.skipif(shutil.which("ffmpeg") is None, reason="ffmpeg not available")
 def test_a_download_yt_dlp_gave_no_duration_stores_the_files_duration(db, monkeypatch, tmp_path):
-    # Facebook reel 257 (E11.5): yt-dlp's info had no duration, the asset stored NULL, and the Library
+    # Facebook reel 257: yt-dlp's info had no duration, the asset stored NULL, and the Library
     # row lost its trim action. Through the real finalize, with a real 3-second file.
     u = _user(db, "fb-duration@example.com")
     a, j = _asset_and_job(db, u, sig="shared", asset_status="downloading", job_status="running")