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_plex_person_bio.py

@@ -1,4 +1,4 @@
-"""E5 S6: person page TMDB bio enrichment. `parse_person` shapes the /person payload (bio + a
+"""Person page TMDB bio enrichment. `parse_person` shapes the /person payload (bio + a
 popularity-ranked, deduped `known_for`); `enrich_person` lazily fetches + persists once (guarded on a
 key + a tmdb_person_id); and /person/{name} serves the bio + resolves `known_for` to library rows.
 DB lane for the endpoint/enrich tests; parse_person is pure. Skipped without Postgres where DB-backed.
@@ -335,7 +335,7 @@ def test_person_detail_serves_bio_and_resolves_known_for(db, monkeypatch, overri
     assert [c["title"] for c in j["known_for"]] == ["Known Movie"]
 
 
-# --- E30 S4: the biography in the reader's language --------------------------------------------------
+# --- the biography in the reader's language -------------------------------------------------------
 # The same /person request appends `translations`; the Hungarian one is kept beside the instance-language
 # biography, and a Hungarian reader is served it, falling back to the English one when TMDB has none.