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/Modal.tsx

@@ -52,8 +52,8 @@ const PANEL = "relative flex flex-col overflow-hidden outline-none panel-scroll-
 
 // Small centered modal shell: backdrop + ESC + scroll-lock close. Each open modal is a layer in the
 // shared `layerStack` so ESC only closes the topmost one — e.g. an error dialog over the tag editor
-// dismisses just the error and returns to the editor underneath, and a player below defers to it
-// (R8 S3). Portals into `document.fullscreenElement ?? document.body`: a node under <body> is painted
+// dismisses just the error and returns to the editor underneath, and a player below defers to it.
+// Portals into `document.fullscreenElement ?? document.body`: a node under <body> is painted
 // beneath whatever holds the fullscreen lock, so a Modal opened from a player in true fullscreen (the
 // download dialog) would otherwise open invisibly.
 export default function Modal({
@@ -85,15 +85,15 @@ export default function Modal({
   // Browser/mouse Back closes the topmost modal instead of navigating away.
   useBackToClose(onClose);
   // ONE container element for the dialog's whole life, re-parented between <body> and the fullscreen
-  // element (deep-review round 4) — and attached before the children commit, so an `autoFocus` child
-  // still gets the focus (round 5). Both rules live in the hook; `ui/AnchoredPopover` shares it.
+  // element — and attached before the children commit, so an `autoFocus` child
+  // still gets the focus. Both rules live in the hook; `ui/AnchoredPopover` shares it.
   const host = useFullscreenPortalHost();
   const panelRef = useRef<HTMLDivElement>(null);
   const rootRef = useRef<HTMLDivElement>(null);
   const titleId = useId();
   // Focus lands in the panel; OWNERSHIP is the whole dialog — the same element the Tab trap below
   // is scoped to, toast host included. Tracking ownership over the panel alone meant tabbing to an
-  // in-dialog toast counted as leaving, and the close-time restore was then skipped (round 6).
+  // in-dialog toast counted as leaving, and the close-time restore was then skipped.
   const bodyRef = useRef<HTMLDivElement>(null);
   const closeRef = useRef<HTMLButtonElement>(null);
   const [footerSlot, setFooterSlot] = useState<HTMLDivElement | null>(null);
@@ -108,11 +108,11 @@ export default function Modal({
   // to take the focus at open, so it lands on the ✕. When the form arrives the focus moves to its
   // first field — but only while it still rests on the ✕: a user who has moved it keeps it there.
   // The form takes it ONCE: arrived while the focus was elsewhere, it disarms all the same, or a later
-  // mutation would pull the focus off a ✕ the user went back to (E32 review round 2). The watch
+  // mutation would pull the focus off a ✕ the user went back to. The watch
   // re-arms only when the focus is taken away, not moved: the body has no control again, or the
   // focused one went (a failed load's Retry giving way to the spinner, or straight to the form) and
   // the focus fell to <body>. It then goes to the ✕, and on to the first field as soon as the body
-  // has one — at once, when other controls stayed beside the one that went (E32 review). Only
+  // has one — at once, when other controls stayed beside the one that went. Only
   // <body>: focus in a dialog stacked on top is not this one's to take.
   useEffect(() => {
     const body = bodyRef.current;