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.

backend/tests/test_formats.py

@@ -1,7 +1,7 @@
-"""build_ydl_opts: EVERY download is clamped to the mp4/H.264 compat profile (E7 browser-
+"""build_ydl_opts: EVERY download is clamped to the mp4/H.264 compat profile (the browser-
 playability guarantee) and must SELECT H.264 (not merely prefer it), so shared downloads play in
 every browser incl. iOS/Safari. Non-mp4 containers and non-H.264 vcodecs are coerced, not honoured.
-See the iPad AV1 broken-player fix + E7 format narrowing."""
+See the iPad AV1 broken-player fix + the format narrowing."""
 
 from app.downloads import edit as editmod
 from app.downloads import formats
@@ -58,7 +58,7 @@ def test_max_height_caps_every_branch():
 
 
 def test_explicit_vcodec_is_clamped_to_the_compat_selector():
-    # E7 browser-playability guarantee: a non-H.264 vcodec no longer opts out — normalize() coerces
+    # Browser-playability guarantee: a non-H.264 vcodec no longer opts out — normalize() coerces
     # vcodec→None, so the avc1 selector (and mp4 merge) is used regardless of what was requested.
     opts = _opts({"mode": "av", "container": "mp4", "vcodec": "vp9"})
     assert opts["format"].startswith("bestvideo[vcodec^=avc1]")