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/client.py

@@ -88,14 +88,14 @@ class PlexClient:
                 "type": item_type,
                 # includeGuids=1 carries each item's Guid array (tmdb/imdb/tvdb ids) in the cheap
                 # listing, so the sync can capture the canonical guid without a per-item rich fetch —
-                # the pairing key for the HU overlay (E5 S5).
+                # the pairing key for the HU overlay.
                 "includeGuids": 1,
                 "X-Plex-Container-Start": start,
                 "X-Plex-Container-Size": size,
             },
         )
         # `totalSize` only: `size` is THIS page's count, and read as the total it made a single
-        # page look like the whole section — "complete" to a prune or a reconcile (E32 review).
+        # page look like the whole section — "complete" to a prune or a reconcile.
         total = mc.get("totalSize")
         return mc.get("Metadata", []) or [], None if total is None else int(total)
 
@@ -142,7 +142,7 @@ class PlexClient:
     def machine_id(self) -> str:
         """Server machineIdentifier — needed in the `uri` of collection add/create calls. Cached.
         Without one the uri would read `server:///…`, a malformed write Plex answers in its own way;
-        it is refused here, by name (E32 S2)."""
+        it is refused here, by name."""
         if not getattr(self, "_machine_id", None):
             mid = self._get("/identity").get("machineIdentifier")
             if not mid: