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.

frontend/src/components/UploadDialog.tsx

@@ -16,7 +16,7 @@ import { DISPLAY_NAME_MAX } from "../lib/api/downloads";
 import { Pressable } from "./ui/Pressable";
 import { Input } from "./ui/Input";
 
-// Local-file ingest (E7 S5c · D5-A). A general upload — ANY file type, no format list: media
+// Local-file ingest. A general upload — ANY file type, no format list: media
 // (video/audio) joins the library with the full function set, a generic file is download-only. The
 // file is streamed to the server via XHR so this dialog can show real upload progress. On success it
 // lands in the Library as an already-`done` job, so we invalidate + point the user there.
@@ -91,7 +91,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
       // with only the transient "connection lost" notice — so this empty catch meant a proxy 502 on
       // a long upload, or an nginx 413, snapped the dialog back to its pre-upload state with nothing
       // named and no job in the list. The comment here claimed the global error modal had it; it
-      // had not since the shared `reportFailure` landed (deep-review round 5).
+      // had not since the shared `reportFailure` landed.
       if (!isAborted(e)) notifyActionError(e, t("downloads.upload.failed"));
       // Re-arm the form so the user can retry or pick again.
       setPct(null);