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/app/logsink.py

@@ -1,4 +1,4 @@
-"""The `log_events` sink (E22 S3): the stored copy of the log the diagnostics console reads.
+"""The `log_events` sink: the stored copy of the log the diagnostics console reads.
 
 stdout stays the complete log (docker keeps 3×50 MB of it); this table keeps the part worth querying
 for `log_retention_days`:
@@ -6,7 +6,7 @@ for `log_retention_days`:
   * every WARNING and above,
   * the `http.request` line of every request at INFO or above (not the DEBUG-level health checks and
     static files),
-  * every line of a request whose user raised their diagnostics level (E22 S5 — `logs.LevelGate`
+  * every line of a request whose user raised their diagnostics level (`logs.LevelGate`
     marks them), which is the detail they consented to have kept,
   * nothing a caller marked `extra={logs.PERSIST: False}` — how the request log caps anonymous
     traffic per client.
@@ -347,8 +347,8 @@ def gc(db: Session, retention_days: int) -> int:
 
 
 def serialize(row: LogEvent) -> dict:
-    """A stored row as the API sends it — one shape for every reader: the admin console (E22.6) and
-    a debug session's evidence list (E16)."""
+    """A stored row as the API sends it — one shape for every reader: the admin console and
+    a debug session's evidence list."""
     return {
         "id": row.id,
         "ts": iso(row.ts),