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_debug_sessions.py

@@ -1,4 +1,4 @@
-"""E16 S1 — debug sessions: Start → reproduce → Stop, the tag on every line, the screenshots.
+"""Debug sessions: Start → reproduce → Stop, the tag on every line, the screenshots.
 
 Four parts:
   * the session's life: one open per user, a second Start joins it, a Stop ends it, the cap ends it
@@ -774,11 +774,11 @@ def test_the_export_is_the_saved_draft_the_evidence_and_the_screenshots(db, make
     assert z.read("screenshot-1.png") == _PNG
 
 
-# --- the issue tracker (S4) -----------------------------------------------------------------
+# --- the issue tracker ----------------------------------------------------------------------
 
 
 class FakeGitea:
-    """Gitea 1.27 as measured (plans/e16 S0): a private repository, one file per asset upload, and
+    """Gitea 1.27 as measured: a private repository, one file per asset upload, and
     a way to make one upload fail."""
 
     def __init__(self):