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/AddToPlaylist.tsx

@@ -16,7 +16,7 @@ import { Input } from "./ui/Input";
 import { SURFACE } from "./ui/surfaces";
 
 // A small anchored menu for toggling a video's membership in the user's playlists, with inline
-// "new playlist" creation. Same shape as the channel tag editor (R8 S1's pilot): radio-style
+// "new playlist" creation. Same shape as the channel tag editor (the popover pilot): radio-style
 // toggles plus a free-text creator, so it follows that file's two conventions — typeahead OFF and
 // the input swallowing non-Escape keys, since character and arrow keys belong to the text field.
 export default function AddToPlaylist({
@@ -55,7 +55,7 @@ export default function AddToPlaylist({
   );
 }
 
-/** The same panel, opened from an object's menu ("Add to playlist…", E28 C1) rather than its own
+/** The same panel, opened from an object's menu ("Add to playlist…") rather than its own
  *  button: anchored to the element the menu came from, and gone when it closes. */
 export function AddToPlaylistAt({
   videoId,