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/issues/gitea.py

@@ -1,5 +1,5 @@
-"""The Gitea issue tracker a debug session's issue goes to (E16 S4) — measured against Gitea 1.27.1
-before a line of this was written (plans/e16-debug-issue.md, S0).
+"""The Gitea issue tracker a debug session's issue goes to — measured against Gitea 1.27.1
+before a line of this was written.
 
 Configured by an admin (sysconfig `issues_gitea_url` / `_repo` / `_token`, the token encrypted at
 rest). The repository must be PRIVATE: an issue carries its reporter's log — page titles, filter
@@ -79,7 +79,7 @@ def target(db: Session) -> dict | None:
 @contextmanager
 def _client(settings: tuple[str, str, str]) -> Iterator[tuple[httpx.Client, str]]:
     # Takes the settings already read, not the session: reading them HERE opened a transaction one
-    # line after `deliver` had ended one, and held it across the calls below (deep-review round 4).
+    # line after `deliver` had ended one, and held it across the calls below.
     url, repo, token = settings
     if not (url and repo and token):
         raise GiteaError("not_configured")
@@ -203,7 +203,7 @@ def deliver(
     # call, so nothing holds a snapshot (and an idle connection) across minutes of tracker I/O.
     # The settings are read HERE and handed to `_client`, not read inside it: reading them there
     # was a SELECT one line after this commit, which reopened the transaction and held it — idle,
-    # with its snapshot pinned — across the repository check and the create (deep-review round 4
+    # with its snapshot pinned — across the repository check and the create (a review
     # measured it in pg_stat_activity). A comment saying otherwise sat right here.
     checkpoint()
     with _client(settings) as (client, repo):
@@ -223,7 +223,7 @@ def deliver(
             )
             # Built ON the state read at the start so nothing already in it is dropped. The claim
             # is NOT among those things, whatever this comment used to say: `set_delivery` strips
-            # any `sending` passed to it and stamps the row's own from `token` (round 4 — removing
+            # any `sending` passed to it and stamps the row's own from `token` (removing
             # the spread left the whole suite green, because the claim never travelled here).
             state = {
                 **state,