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/DataTable.progressive.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E31 S4d: the channel manager's "All" page (~400 rows) blocked one frame for 0.7–0.9 s on prod to
+// The channel manager's "All" page (~400 rows) blocked one frame for 0.7–0.9 s on prod to
 // mount, and every key in its search re-rendered every row. A long page now mounts its first rows at
 // once and the rest in small background steps (progressiveRows), and a step leaves the rows it holds
 // alone (a row is its own component, whose cells the React Compiler caches).
@@ -101,7 +101,7 @@ describe("DataTable — a long page mounts in steps", () => {
     await waitFor(() => expect(bodyRows()).toBe(150));
   });
 
-  // E31 S5: Channels turns the page to the row a priority pick moved and focuses it in the commit
+  // Channels turns the page to the row a priority pick moved and focuses it in the commit
   // that follows. That row was at index 40 of a 50-row page: not mounted yet, focus fell to <body>.
   it("mounts a revealed row in the render that turns to its page", () => {
     host = document.createElement("div");
@@ -125,7 +125,7 @@ describe("DataTable — a long page mounts in steps", () => {
     expect(host.querySelector("tbody")?.textContent).toContain("r 90");
   });
 
-  // E31 review 2: the reveal floor held for the page's lifetime. Channels never clears its `follow`, so
+  // The reveal floor once held for the page's lifetime. Channels never clears its `follow`, so
   // after one priority pick every later sort or search mounted every row up to that channel at once.
   it("lets a revealed row go once the reader moves to another view", () => {
     host = document.createElement("div");