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/App.tsx

@@ -146,9 +146,9 @@ export default function App() {
   // account can reach (the nav rail's own list, plus Settings, which is not in the rail's order).
   useGlobalKeymap(!!me, setPage, me ? [...moduleOrder(me), "settings"] : []);
 
-  // The account's consented diagnostics level (E22 S5), applied to this tab's log: raised, the log is
+  // The account's consented diagnostics level, applied to this tab's log: raised, the log is
   // kept in more detail and streamed to the server until the level lapses. A debug session that is
-  // recording (E16) raises it for its own run and tags what it keeps. A lapse (its own timer, or the
+  // recording raises it for its own run and tags what it keeps. A lapse (its own timer, or the
   // server saying it is off) refetches `me`, so Settings and the recorder show what is in force.
   // Re-running this on every `me` refetch is FREE and therefore the deps are the honest ones:
   // applyDiagnostics computes an effective-level key and returns early when it has not moved
@@ -395,7 +395,7 @@ export default function App() {
       )}
       {/* `@container/appmain`: the floating header + its search box adapt to THIS column's width
           (which shrinks when the filter sidebar is open), not the viewport — a viewport breakpoint
-          can't see the sidebar-driven narrowing (E5 S7). NB: `container-type: inline-size` implies
+          can't see the sidebar-driven narrowing. NB: `container-type: inline-size` implies
           `contain: layout`, making this column a containing block for `position: fixed` descendants
           too — safe today because floating-ui menus render via FloatingPortal (to document.body) and
           the toasts below are absolute; keep any new fixed overlay inside here body-portalled. */}
@@ -456,7 +456,7 @@ export default function App() {
           "take me to X" the same way. Their keys are bound by useGlobalKeymap above. */}
       <KeyboardHelp />
       <AppCommandPalette />
-      {/* The debug recorder (E16): its start dialog, and its pill docked under the toasts. */}
+      {/* The debug recorder: its start dialog, and its pill docked under the toasts. */}
       {me && <DebugRecorder />}
       <Suspense fallback={null}>
         {aboutOpen && (