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/useDownloadActions.tsx

@@ -26,7 +26,7 @@ import { jobTitle } from "./downloadTitle";
 import { type ActionMenuItem } from "./ui/ActionMenu";
 import { menuGroups } from "./ui/menuGroups";
 
-// A download's ONE action list (E28 C1): the row's "…", its right-click, long-press and Shift+F10,
+// A download's ONE action list: the row's "…", its right-click, long-press and Shift+F10,
 // on the queue, the library and the shared tab. The row keeps its primary action (play; pause /
 // resume / retry in the queue), the save link and its state (the error and its details) in sight;
 // a library row also keeps Share (the link copy) in sight, ahead of save. The rest — the editors,
@@ -81,7 +81,7 @@ export function useDownloadActions({
     if (await confirm({ title, message: body, confirmLabel: action, danger: true })) fn();
   };
 
-  // One-click share (E7 S3 · D2-A): ensure a downloadable link, copy it, toast with "open in new tab"
+  // One-click share: ensure a downloadable link, copy it, toast with "open in new tab"
   // (see quickShareLink). Re-choosing reuses the existing link; a per-job in-flight guard also stops a
   // double pick from racing two POSTs into two links before the cache is primed. Advanced controls
   // (password, expiry, revoke, share-with-a-user) live in the Share dialog, "Manage sharing".
@@ -233,7 +233,7 @@ export function useDownloadActions({
   }
 
   function sharedItems(job: DownloadJob): ActionMenuItem[] {
-    // Same media_kind + can_download gate as the library, via the shared helper (S5b).
+    // Same media_kind + can_download gate as the library, via the shared helper.
     const a = sharedRowActions(job);
     return menuGroups(
       [...(a.play ? [play(job)] : []), ...(a.save ? [save(job)] : [])],