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_watch_sync.py

@@ -1,4 +1,4 @@
-"""S2 watch-sync semantics: Plex is the source of truth.
+"""Watch-sync semantics: Plex is the source of truth.
 
 Two independent pieces:
 - `_pull_apply` (DB lane): on a genuine divergence for an item Plex knows about, Plex now WINS —
@@ -45,7 +45,7 @@ class _FakePlex:
 
 
 def test_pull_plex_wins_over_diverging_synced_local_resume(db):
-    """The core S2 change: a SYNCED local row (its state already reached Plex) that later diverges from
+    """The core change: a SYNCED local row (its state already reached Plex) that later diverges from
     Plex = a real external Plex-side change, so the incremental pull resolves to PLEX's position — Plex
     is authoritative, no LWW. (A DIRTY local row is protected instead — see the dirty-guard test.)"""
     item_id, user_id = _seed_item_and_user(db)
@@ -210,7 +210,7 @@ def test_pull_settles_flag_on_matching_dirty_row(db):
 
 
 def test_movie_probe_flips_stale_in_progress_to_watched(db):
-    """S1 core: a local in-progress MOVIE that Plex has since finished (viewCount>0) and dropped from
+    """Core: a local in-progress MOVIE that Plex has since finished (viewCount>0) and dropped from
     Continue-Watching flips to watched on the incremental pass — without waiting for the daily reconcile.
     This is the "movie stuck in progress after scrubbing the credits" bug."""
     item_id, user_id = _seed_item_and_user(db)