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_discover.py

@@ -1,4 +1,4 @@
-"""E24 S1 — the Home surface's read-model.
+"""The Home surface's read-model.
 
 Covers the four things that can silently rot here: the time-budget bands (pure), the cross-source
 resume rail's merge order, the hero's tier ladder, and the comfort shelf (its schema-level one-of
@@ -632,7 +632,7 @@ def test_a_pinned_video_survives_leaving_the_feed_scope(db):
 
 def test_the_plex_surprise_obeys_the_time_budget(db):
     """The one control on the page has to mean what it says: a 100-minute film is not an answer to
-    "10 perc". The YouTube half of this rule has had its own test since S1; the Plex half's duration
+    "10 perc". The YouTube half of this rule has long had its own test; the Plex half's duration
     bounds could be deleted with the whole suite still green, because every test that reached
     `plex_surprise` passed budget="any"."""
     user, _ = _seed_yt(db)
@@ -923,7 +923,7 @@ def test_pinning_a_plex_show_resolves_to_the_episode_that_would_play(db, as_user
 
 def test_pinning_a_plex_movie_plays_itself_without_a_cycle(db, as_user):
     """The self-reference that once 500'd the whole shelf, on the other leaf kind: `next` has to be a
-    distinct dict, not the tile aliased into itself. Covered for a video since S1, never for a movie."""
+    distinct dict, not the tile aliased into itself. Long covered for a video, never for a movie."""
     user, _ = _seed_yt(db)
     _seed_plex(db, user)
     _enable_plex(db)
@@ -941,7 +941,7 @@ def test_the_reroll_endpoint_parses_its_whole_exclusion_list_and_its_press_count
     hero comes back.
 
     NOT the `MAX_EXCLUDE` clamp, which this sends three keys against a cap of 32 and cannot see. The
-    docstring used to claim it did (round 2). The clamp is deliberately left unpinned: it guards
+    docstring used to claim it did. The clamp is deliberately left unpinned: it guards
     nothing the route would otherwise do — see the constant's own comment — and a test that sent 33
     keys could only assert that the request still answers, which the comma split already proves."""
     user, _ = _seed_yt(db)
@@ -975,7 +975,7 @@ def test_pinning_something_that_does_not_exist_is_a_404(db, as_user):
 
 def test_a_pin_whose_target_vanishes_mid_request_is_a_404_not_a_500(db, as_user, monkeypatch):
     """`add_pin`'s `except IntegrityError` arm was reached by no test at all, so the 500→404
-    narrowing could be reverted with the whole suite green (round 2). The 404 above comes from the
+    narrowing could be reverted with the whole suite green. The 404 above comes from the
     `resolve_ref` PRE-CHECK, a different branch that returns before the INSERT.
 
     The race the arm exists for — a Plex sync dropping the row between `resolve_ref` confirming it
@@ -1091,7 +1091,7 @@ def test_two_overlapping_reorders_never_share_a_slot(db, as_user):
     row — two pins on one slot (deep review, debt burn). Locked, the second waits for the first and
     deals from its result: here a partial list (a tab with Plex off, one pin hidden), whose left-out
     pin must stay where the FIRST reorder put it — a lock taken in position order handed the waiter
-    fresh rows in its stale order, and it dealt that pin back to its old slot (round 2)."""
+    fresh rows in its stale order, and it dealt that pin back to its old slot."""
     import threading
 
     from app.db import SessionLocal