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_credentials.py

@@ -1,4 +1,4 @@
-"""S6 — per-user, per-host download cookies: host-scope, cache privacy, encryption, write-only API.
+"""Per-user, per-host download cookies: host-scope, cache privacy, encryption, write-only API.
 
 The two load-bearing security properties are covered as behaviour, not prose:
   * **cache privacy** — a cookie-authed download is a PRIVATE, salted asset; a second user without a
@@ -283,7 +283,7 @@ class TestResolveCookiefile:
 
 
 # --- Retry / Re-download take a cookie added later (DB) -----------------------------------------
-# Measured on dev (E11.5): a Facebook reel failed on the login wall, the user added their cookie as
+# Measured on dev: a Facebook reel failed on the login wall, the user added their cookie as
 # the failure copy tells them to, pressed Retry — and it failed again, because the job still held the
 # shared asset the worker never attaches a cookie to. Re-download forked a salt the gate could not
 # recognise, so even a job downloaded WITH a cookie rebuilt without it.