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/ui/Tabs.tsx

@@ -9,12 +9,12 @@ import CountBadge from "./CountBadge";
 // Below it a side rail leaves a phone too little width for the content beside it (measured at 390 px:
 // the rail clipped its own labels and Settings → Account pushed its buttons over their text), so there
 // it is a horizontal row that scrolls. A page whose sub-tab is worth keeping persists it with
-// lib/storage's useTabPersistedState (per browser tab, E31 S4b: Admin Users, Config, Settings, Stats):
+// lib/storage's useTabPersistedState (per browser tab: Admin Users, Config, Settings, Stats):
 // a reload keeps the user where they were, and a fresh tab opens on the first sub-tab. Not every
 // caller: the channel manager's view is account-wide (ChannelsProvider), and Downloads, a channel page
 // and the player settings start on their first tab each time.
 //
-// An ARIA tab list (E28 review F3), not a row of toggle buttons: `tablist` / `tab` / `aria-selected`,
+// An ARIA tab list, not a row of toggle buttons: `tablist` / `tab` / `aria-selected`,
 // ONE Tab stop (the selected tab; roving tabindex), the arrows move and select (either axis, since the
 // vertical rail is a row below `sm`), Home/End jump to the ends. The panel it controls is wired with
 // `useTabPanel`: the tabs point at it (`aria-controls`) and it is labelled by the selected tab.
@@ -27,7 +27,7 @@ export interface TabDef {
   badge?: number; // optional count pill (e.g. pending requests); hidden when 0/undefined
   icon?: LucideIcon; // optional leading icon (the vertical settings rail)
   testId?: string; // optional data-testid, to preserve existing e2e hooks
-  /** The vertical rail's heading this tab sits under (E31: Configuration's sixteen groups under four).
+  /** The vertical rail's heading this tab sits under (Configuration's sixteen groups under four).
    *  Drawn above the first tab of each run; tabs of one heading stay together. */
   group?: string;
 }