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/mediaPicture.contract.test.ts

@@ -10,7 +10,7 @@ import {
   mediaPictureBox,
 } from "./ui/mediaCardStyles";
 
-// E31 S5, measured: at text size 1.3 a 242px feed thumbnail put the 63px Play 13px into the 37px hover
+// Measured: at text size 1.3 a 242px feed thumbnail put the 63px Play 13px into the 37px hover
 // row (the Play and the row grow with the root font, the picture's height does not). A narrow picture
 // (under 14rem, so the cut moves with the text size) centres what it carries above the row instead.
 describe("a media picture", () => {
@@ -28,7 +28,7 @@ describe("a media picture", () => {
     expect(layer.split(" ")).toContain("@max-[14rem]/pic:pb-(--bar-room)");
   });
 
-  // E31 review 2: the lift applied to every narrow picture, and the feed's row and tile views (a
+  // The lift once applied to every narrow picture, and the feed's row and tile views (a
   // 10rem thumbnail, no hover row) showed their Play 1.25rem above centre over an empty band. The band
   // is now the row's own: set only by a box that holds one.
   it("lifts it only where a hover row is there to clear", () => {