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_subtitle_prepare.py

@@ -12,7 +12,7 @@ thread sat with it), and picking the second language cost the whole read again.
 
 These tests pin the three things that fixes it: the request never waits for ffmpeg, the extraction
 publishes a window around the playhead before the whole track, and one ffmpeg run covers every
-embedded text track — plus what the deep-review of that fix found (cues doubled at the window/full
+embedded text track — plus what the review of that fix found (cues doubled at the window/full
 seam, a truncated pass cached as complete, a poll that respawned ffmpeg every second, a sidecar stuck
 behind an unrelated pass). No Postgres (the auth half is in test_plex_subtitle_auth.py, which needs
 the DB). Almost every test stubs ffmpeg out, but not all: a few run a real child process, and
@@ -316,7 +316,7 @@ def test_a_cached_vtt_from_an_older_build_counts_as_complete(tmp_path):
     assert subtitles.read_meta(key) == ([[0.0, 1_000_000_000.0]], True)
 
 
-# --- the deep-review round: the seams between the window and the full pass ----------------------
+# --- the seams between the window and the full pass ---------------------------------------------
 
 
 def test_a_window_and_the_full_pass_do_not_double_a_cue():
@@ -614,7 +614,7 @@ def test_an_absurd_position_is_rejected_before_it_reaches_ffmpeg(client, monkeyp
     assert pool.jobs == []
 
 
-# --- the second deep-review round ---------------------------------------------------------------
+# --- failed passes, seeks, saturation and shutdown ----------------------------------------------
 
 
 def _window_raises(monkeypatch):
@@ -865,7 +865,7 @@ def test_a_full_pass_resumed_mid_file_lines_up_on_a_real_file(tmp_path, monkeypa
     assert merged.count("-->") == 30, "a cue across the join was kept twice or lost"
 
 
-# --- the third deep-review round ----------------------------------------------------------------
+# --- transient failures, read-ahead, memos and slot fairness ------------------------------------
 
 
 def test_a_transient_failure_is_said_but_is_not_a_final_answer(monkeypatch):
@@ -1094,7 +1094,7 @@ def test_a_nul_rating_key_names_no_row():
     assert by_rating_key(None, PlexItem, "12\x0034") is None  # type: ignore[arg-type]
 
 
-# --- the fourth deep-review round ---------------------------------------------------------------
+# --- output that could not be kept, absurd keys -------------------------------------------------
 
 
 def test_a_pass_whose_output_could_not_be_kept_is_backed_off_like_a_failed_one(monkeypatch):
@@ -1154,7 +1154,7 @@ def test_an_absurd_rating_key_or_ordinal_is_refused_not_a_500(client, monkeypatc
     # ENAMETOOLONG rather than the real filesystem: on Linux pathlib re-raises it (the defect), but
     # Windows reports ERROR_INVALID_NAME, which pathlib's `exists()` already swallows — so on a dev
     # machine this assertion passed with `_exists` reverted and only the container lane was honest
-    # about it (deep-review round 5).
+    # about it.
     def _too_long(self):
         raise OSError(errno.ENAMETOOLONG, "File name too long")