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_feed_relevance_window.py

@@ -1,4 +1,4 @@
-"""Relevance ranks only the newest RANK_WINDOW matches (E31), DB-lane.
+"""Relevance ranks only the newest RANK_WINDOW matches, DB-lane.
 
 A broad word ("a") matched most of the prod library and ts_rank read every match's tsvector: 6.8 s a page.
 The ranking now covers the newest RANK_WINDOW matches and the rest follow newest first. The window's start
@@ -126,7 +126,7 @@ def test_cursor_walk_matches_the_unpaged_order(db, as_user, monkeypatch, window,
 )
 def test_a_hand_made_relevance_cursor_is_refused_not_a_crash(db, as_user, payload):
     # The page halves compare the cursor's timestamp with the window's in Python: a null or a naive
-    # one raised there, past the route's 400 — a 500 (E31 review 2).
+    # one raised there, past the route's 400 — a 500.
     client = as_user(_seed(db))
     cursor = base64.urlsafe_b64encode(json.dumps(payload).encode()).decode()
     resp = client.get(f"/api/feed?q=foo&sort=relevance&limit=2&cursor={cursor}")