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_search_scrape.py

@@ -1,4 +1,4 @@
-"""youtube/search_scrape — the zero-quota InnerTube search (E32 S2: the module was 20% covered).
+"""youtube/search_scrape — the zero-quota InnerTube search (the module was 20% covered).
 
 Everything here breaks silently when YouTube moves: a parser that stops finding videos, a cursor that
 stops paging, a fallback key that outlives its usefulness. The network is an httpx MockTransport.
@@ -168,7 +168,7 @@ def test_the_live_key_is_fetched_once_and_cached(net):
 def test_a_failed_key_fetch_falls_back_for_now_and_tries_again_next_time(net, results_html):
     """The fallback pair used to be CACHED for the life of the process: one failed fetch pinned
     every later search to a hard-coded key, and once that went stale search returned nothing,
-    with no error anywhere (E32 S2)."""
+    with no error anywhere."""
     net["handler"] = _serve(results_html=results_html)
     search_scrape.search("a")
     assert net["calls"][-1].url.params["key"] == search_scrape._FALLBACK_KEY