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/detail.py

@@ -153,7 +153,7 @@ def show_detail(
         all_cards.append(card)
     show_roll = rollup.rollup(all_cards)
 
-    # Cast + IMDb + tagline are served from the DB row (persisted by the cast backfill, E5 S5b). The one
+    # Cast + IMDb + tagline are served from the DB row (persisted by the cast backfill). The one
     # remaining live call is `related` — a Plex recommendation hub we don't mirror — which also carries
     # the metadata used to lazily fill a not-yet-enriched show. Any live failure just drops `related` and
     # the lazy fill; the row-served detail below still renders (outage-resilient).
@@ -213,7 +213,7 @@ def show_detail(
             "first": show_roll["first"],
             "episode_count": show_roll["episode_count"],
             "collection_keys": sh.collection_keys or [],
-            # TMDB enrichment (E5 S2), served from the row.
+            # TMDB enrichment, served from the row.
             "tmdb": plex_tmdb.serve_block(db, sh, lang=lang),
         },
         "seasons": [_season_block(se, by_season.get(se.id, [])) for se in seasons],
@@ -353,7 +353,7 @@ _TRACK_TAIL = re.compile(r"\(([^()]+)\)\s*$")
 
 
 def _track_naming(s: dict) -> dict:
-    """What the player names a track from (E30). Plex's `displayTitle` is in the SERVER's language
+    """What the player names a track from. Plex's `displayTitle` is in the SERVER's language
     ("English (AC3 5.1)", "magyar Forced", "中文 (EAC3 Stereo)"), so it is sent only as the fallback
     `label`; the client builds the name in the reader's language from the language tag, the
     forced / SDH flags and the codec tail. No title and no language → no label (the client numbers it)."""
@@ -538,6 +538,6 @@ def item_detail(
         "prev_id": prev_id,
         "next_id": next_id,
         "collections": _collection_strips(db, it, user.id, lang),
-        # TMDB enrichment (E5 S2), served from the row — movies only (episodes aren't enriched).
+        # TMDB enrichment, served from the row — movies only (episodes aren't enriched).
         "tmdb": plex_tmdb.serve_block(db, it, lang=lang) if it.kind == "movie" else None,
     }