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/VideoCard.actions.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E28 C1: the card shows its state and the everyday toggles (watched, save, add to playlist) and a
+// The card shows its state and the everyday toggles (watched, save, add to playlist) and a
 // "…" into the list's ONE menu; download, details, hide and reset live only in that menu. Every view
 // carries the same set, and a view that shows the "…" twice (the card: in the hover row on the
 // picture, and beside the title for touch — only one is displayed per device) points both at the
@@ -92,7 +92,7 @@ describe.each(Object.keys(FEED_VIEW_SPEC) as FeedView[])("the %s view's actions"
   });
 });
 
-// E28 S3h, the owner's pick B (2026-09-26): the compact row had a leading action column that sat
+// The owner's pick (2026-09-26): the compact row had a leading action column that sat
 // empty at rest. Now the title leads, the "…" is always at the row's end, and on hover the quick
 // toggles float in beside it over the date, which steps aside.
 describe("the compact row (rowsCompact)", () => {
@@ -116,7 +116,7 @@ describe("the compact row (rowsCompact)", () => {
 });
 
 // On a shelf (Home's strips) the card is one title line over the channel alone, so every card on
-// the strip is the same height (E28 S3h, the owner's shelf density).
+// the strip is the same height (the owner's shelf density).
 describe("the card on a shelf", () => {
   function renderSmall(shelf: boolean) {
     render(