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/rollup.py

@@ -13,10 +13,10 @@ from app.models import PlexItem, PlexState
 # Sort sentinel for a rail row whose timestamp is missing, so `key=lambda r: r[0] or NO_TS` reads the
 # same at every rail that merges two type-homogeneous lists in Python.
 #
-# It is NOT a crash fix, whatever the E24 commit that introduced it says. The `(ts is not None, ts)`
+# It is NOT a crash fix, whatever the commit that introduced it says. The `(ts is not None, ts)`
 # key it replaced cannot raise: tuple comparison finds the first UNEQUAL element via `==`, so two
 # `(False, None)` keys compare equal and `None < None` is never reached — verified by execution
-# during the E24 epic-end review, over every permutation of null/non-null rows. What the sentinel
+# during a review, over every permutation of null/non-null rows. What the sentinel
 # buys is one idiom instead of two, and a key that says "missing sorts last" outright rather than
 # encoding it in a boolean flag. A BARE `r[0]` would genuinely raise, which is the mutation the
 # rails' tests do guard against.