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/conftest.py

@@ -1,4 +1,4 @@
-"""Pytest fixtures for the DB-backed integration lane (R6 S3b).
+"""Pytest fixtures for the DB-backed integration lane.
 
 The bulk of the suite is pure-logic and needs no database. This adds an OPT-IN `db` fixture
 that hands a test a real session against a throwaway Postgres (the app's own engine, bound to
@@ -38,8 +38,8 @@ from app.models import User
 _MISSING = object()
 
 # A would-be silent skip (no DB, or no ffmpeg) is a hard error unless the run says otherwise — the
-# suite must not print green while the DB-backed / ffmpeg tests were quietly not run. ON by default
-# since E32: as an opt-in only the compose lane set, a hand-run `pytest` skipped ~60 DB tests
+# suite must not print green while the DB-backed / ffmpeg tests were quietly not run. ON by default:
+# as an opt-in only the compose lane set, a hand-run `pytest` skipped ~60 DB tests
 # and still passed. `SIFTLODE_TEST_REQUIRE_DB=0` is the explicit "pure-logic only" run.
 _REQUIRE_FULL = os.getenv("SIFTLODE_TEST_REQUIRE_DB", "1") != "0"
 
@@ -117,7 +117,7 @@ def _request_dep(db, unfinished: list[str]):
     savepoint released inside the handler commits nothing. The test's own staged rows are flushed before the request starts — they are the
     test's business, not the handler's. A request that raises is rolled back, as production's
     `close()` rolls it back: without that, what a failing handler flushed but never committed stayed
-    readable to the rest of the test — the one state production cannot be in (E16: an issue created
+    readable to the rest of the test — the one state production cannot be in (an issue created
     on the tracker before a failed upload was kept only by an explicit commit, and nothing saw it
     missing)."""