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/ShareDialog.tsx

@@ -118,10 +118,10 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
         <IconButton label={t("downloads.share.copy")} size="sm" variant="ghost" onClick={copy}>
           {copied ? <Check className="w-4 h-4 text-success" /> : <Copy className="w-4 h-4" />}
         </IconButton>
-        {/* All three on the same rung. Only Copy was migrated in S1, which left one 2rem button
+        {/* All three on the same rung. Only Copy was migrated at first, which left one 2rem button
             between two 1.75rem ones with a different radius and hover fill — and, on a coarse
             pointer, `.touch-target` grew ONLY the migrated one to 2.75rem while the destructive
-            Revoke stayed small (E10 review round 1). */}
+            Revoke stayed small. */}
         <IconButton
           label={
             link.has_password
@@ -136,7 +136,7 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
           }}
           // On the ICON, not the button: `clsx` does not merge Tailwind, so a colour className loses
           // to the variant's own `text-muted`/`hover:text-fg` by stylesheet order and the state was
-          // simply invisible (E10 review round 2). The Copy button above already does it this way.
+          // simply invisible. The Copy button above already does it this way.
         >
           <Lock className={`w-4 h-4 ${link.has_password ? "text-accent" : ""}`} />
         </IconButton>
@@ -235,7 +235,7 @@ function LinkShare({ job }: { job: DownloadJob }) {
     queryFn: () => api.downloadLinks(job.id),
   });
   const [creating, setCreating] = useState(false);
-  // D1 (E7 S3): downloadable by default — matches the one-click Share (which relies on the backend
+  // D1: downloadable by default — matches the one-click Share (which relies on the backend
   // default) and the common "let them have the file" intent. The owner can uncheck it here.
   const [allowDownload, setAllowDownload] = useState(true);
   const [expiryDays, setExpiryDays] = useState<number | null>(null);