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/app/plex/watch_sync.py

@@ -11,7 +11,7 @@ already pages) and upsert them into the owner's `plex_states`. Plex wins on over
 exist only in Siftlode are left untouched (union, Plex-authoritative on the intersection).
 
 Phases B (Siftlode→Plex push via scrobble/timeline) and C (incremental Plex→Siftlode via the watch
-history + onDeck) build on this and land in later ships. As of S2 Plex is the source of truth: the
+history + onDeck) build on this and land in later ships. Plex is now the source of truth: the
 player pushes the resume position back to Plex on the periodic checkpoint (not just the final flush),
 and the incremental pull resolves a genuine divergence to Plex's state (see `_pull_apply`) rather than
 running a last-write-wins comparison.
@@ -81,7 +81,7 @@ def _scan_plex_states(
     The settled keys matter to the reconcile alone: it reads "absent from the scan" as "Plex
     un-watched it", which a walk cut short by a Plex hiccup does not mean (see `_paginate`). Settled
     is decided PER SECTION: one section that never settles (a library emptied on Plex while its
-    mirrored items are kept) must not stop un-watches syncing in all the others (E32 review)."""
+    mirrored items are kept) must not stop un-watches syncing in all the others."""
     item_id_by_rk = _rk_to_id(db)
     wanted = _enabled_section_keys(db)
     # The sections we mirror something from: a total of 0 from one of those contradicts the mirror.
@@ -117,7 +117,7 @@ def _scan_plex_states(
                 out[item_id] = res
         # A section the server SAYS is empty (total 0, nothing listed) settles too. Believed only for
         # a section we mirror nothing from: one whose items we hold answering 0 is a Plex that lost
-        # the library for a moment, not one that un-watched all of it (E32 review) — so it is left
+        # the library for a moment, not one that un-watched all of it — so it is left
         # unsettled, and its rows are not cleared.
         listed = scanned > before
         said_empty = progress.get("total") == 0 and not listed and key not in mirrored
@@ -402,7 +402,7 @@ def _pull_apply(
         if st is not None and not st.synced_to_plex:
             st.synced_to_plex = True  # already matches Plex → just settle the flag, no churn
         return None
-    # Plex-authoritative (S2): on a genuine divergence for an item Plex knows about, Plex wins — no
+    # Plex-authoritative: on a genuine divergence for an item Plex knows about, Plex wins — no
     # last-write-wins tug-of-war. This is safe because prompt write-back (playback.py pushes the resume
     # position to Plex on the ~10s checkpoint, not just on the final flush) keeps Plex current, so a
     # local edit that's "newer" than Plex has almost always already been pushed and matched above by
@@ -620,9 +620,8 @@ def _clear_unwatched(db: Session, user_id: int, plex_states: dict, settled: set[
     """Clear the user's rows Plex no longer has a record for — the mirrored ones (Plex un-watched
     them); hidden and dirty rows stay (a dirty one is re-pushed by `_repush_dirty`). Only rows of the
     sections the scan SETTLED: a walk a Plex hiccup cut short leaves out rows it merely failed to
-    reach, and clearing those un-watched them here until the next complete run (E32 S2); an item of
-    a library the allowlist leaves out is absent from every scan, and was cleared on every run (E32
-    review)."""
+    reach, and clearing those un-watched them here until the next complete run; an item of
+    a library the allowlist leaves out is absent from every scan, and was cleared on every run."""
     n = 0
     rows = db.execute(
         select(PlexState)