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.py

@@ -1,4 +1,4 @@
-"""DB-backed regression (E5 S4): GET /api/plex/person/{name} returns a cast/crew member's
+"""DB-backed regression: GET /api/plex/person/{name} returns a cast/crew member's
 filmography — movies + shows in the ENABLED libraries whose cast OR directors include the name,
 each carrying the caller's watch-state, plus the cached headshot. Also covers `content_libs`
 scoping (disabled libraries excluded) and the kind filter (episodes are not movies).
@@ -123,7 +123,7 @@ def test_person_detail_unknown_name_is_empty(db, monkeypatch, override_dep):
     resp = _get(db, user, monkeypatch, override_dep, "/api/plex/person/Ghost%20Actor")
     assert resp.status_code == 200, resp.text
     body = resp.json()
-    # An unknown name has no plex_people row → no bio, no titles (E5 S6 added the bio fields, all null).
+    # An unknown name has no plex_people row → no bio, no titles (the bio fields are all null).
     assert body == {
         "name": "Ghost Actor",
         "photo": None,