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_reconcile.py

@@ -1,5 +1,5 @@
 """The daily full watch reconcile (`run_plex_watch_reconcile`) — the one pass that DELETES watch state:
-a mirrored row Plex no longer has a record for is read as "Plex un-watched it" and cleared. E32 S2
+a mirrored row Plex no longer has a record for is read as "Plex un-watched it" and cleared. A review
 found it trusting a scan a Plex hiccup had cut short; these hold what it may and may not clear.
 """
 
@@ -149,7 +149,7 @@ _LISTED = [
 
 def test_a_genuinely_empty_second_section_still_lets_the_reconcile_clear(db, monkeypatch, world):
     # The server SAYS the shows section holds 0: that is a complete listing, not a hiccup — before
-    # the E32 review it read as "incomplete" forever and un-watches never synced again.
+    # a review it read as "incomplete" forever and un-watches never synced again.
     state, rows = world
     _local_states(state)
     db.commit()
@@ -181,8 +181,8 @@ def test_a_section_we_mirror_that_says_0_keeps_its_rows_and_does_not_hold_up_the
     db, monkeypatch, world
 ):
     # A total of 0 from a section our items come from is a Plex that lost the library for a moment,
-    # not one that un-watched all of it (E32 review round 2): its rows stay. And it is decided per
-    # section: before round 3 one such section (a library emptied on Plex, its mirrored items kept)
+    # not one that un-watched all of it: its rows stay. And it is decided per
+    # section: at first one such section (a library emptied on Plex, its mirrored items kept)
     # made EVERY run incomplete, and un-watches in every other section never synced again.
     state, rows = world
     _local_states(state)
@@ -196,7 +196,7 @@ def test_a_section_we_mirror_that_says_0_keeps_its_rows_and_does_not_hold_up_the
 
 def test_a_library_the_allowlist_leaves_out_keeps_its_watch_states(db, monkeypatch, world):
     # Its items are absent from every scan because the scan never walks it — not because Plex
-    # un-watched them. It cleared them on every run (E32 review round 2).
+    # un-watched them. It cleared them on every run.
     state, _rows = world
     other = PlexLibrary(plex_key="2", title="Shows", kind="show", enabled=True)
     db.add(other)