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/plex/imgcache.py

@@ -26,7 +26,7 @@ _CT_EXT = {ct: ext for ext, ct in _EXT_CT}
 # Target box per variant — Plex resizes to these server-side. The full poster (400px) covers a detail
 # page's ≤224px poster at 2× DPR, the full art (1280px) a hero backdrop. A card is much smaller: a grid
 # poster is ~144px and a shelf's landscape art ~212px, so the `_sm` pair serves those at 1.33× DPR
-# (E31 S4e: cards fetched the 400 poster, 42 KB, and the 1280 art for a 212px picture). The browser
+# (cards used to fetch the 400 poster, 42 KB, and the 1280 art for a 212px picture). The browser
 # picks between a pair itself (`srcset`, built by the frontend's `lib/plexImage` cardImage, whose
 # widths plexImage.contract.test holds to these).
 SIZES = {