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/lib/plexStats.test.ts

@@ -25,7 +25,7 @@ describe("plexStats formatters", () => {
     expect(fmtBitrate(820_000, "en")).toBe("820 kbps");
     expect(fmtBitrate(4_200_000, "en")).toBe("4.2 Mbps");
     expect(fmtBitrate(1_000_000, "en")).toBe("1.0 Mbps");
-    expect(fmtBitrate(4_200_000, "hu")).toBe("4,2 Mbps"); // E30 S3: the UI language's decimal mark
+    expect(fmtBitrate(4_200_000, "hu")).toBe("4,2 Mbps"); // the UI language's decimal mark
   });
 
   it("fmtBufferSecs shows one decimal, dashes negatives/non-finite", () => {
@@ -161,7 +161,7 @@ describe("fmtMode", () => {
 // believe. It was reading the master playlist's BANDWIDTH, and for a remux ffmpeg computes that from
 // the only stream whose rate it knows — the `-b:a 192k` AAC track — plus 10%: 192000 × 1.1 = 211200,
 // a constant that has nothing to do with the video. And the panel never said which audio or subtitle
-// track was playing, though the player has switched both since E5.
+// track was playing, though the player can switch both.
 
 describe("measured bitrate", () => {
   it("reports bytes per second of MEDIA, not of wall clock", () => {