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/plexSubUrl.ts

@@ -19,7 +19,7 @@ export type Sub =
   | { kind: "collection"; id: string }; // the Collection info page (id = collection rating_key)
 
 // Mirror the Plex sub-view to ?v=<kind>:<args> so a show / season / movie-info / person / playlist page
-// — and the flat browse grid — are bookmarkable. Discover is the ROOT landing (E5 S4), so it (and the
+// — and the flat browse grid — are bookmarkable. Discover is the ROOT landing, so it (and the
 // ephemeral player) is not mirrored (encode → null); the grid gets `?v=browse` so "Browse all" is a
 // distinct, shareable URL. The person name is the whole tail after the first ":", so a name containing
 // ":" round-trips.
@@ -51,7 +51,7 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
   },
   // `fromStart` is the Restart CLICK, not a property of the entry: kept in history.state it restarted
   // the item from 0 again on every reload or Forward onto the player, and the progress checkpoint
-  // then overwrote the resume point it had skipped (E28 review E1).
+  // then overwrote the resume point it had skipped.
   entry: (v) => (v.kind === "player" && v.fromStart ? { ...v, fromStart: undefined } : v),
   decode: (raw) => {
     const i = raw.indexOf(":");