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/sysconfig.py

@@ -181,7 +181,7 @@ SPECS: tuple[ConfigSpec, ...] = (
     # --- Audit log ---
     # Days to keep admin audit-log rows before the audit_gc job prunes them (0 = keep forever).
     ConfigSpec("audit_retention_days", "int", "audit", "audit_retention_days", min=0, max=3650),
-    # --- Diagnostics (E22) ---
+    # --- Diagnostics ---
     # Runtime log level for the API process. Applied without a restart — see routes/config.py and
     # main.py's lifespan.
     ConfigSpec("log_level", "choice", "diagnostics", "log_level"),
@@ -198,7 +198,7 @@ SPECS: tuple[ConfigSpec, ...] = (
     # (plex_sync_interval_min is tuned from the Scheduler dashboard, not here — it's a scheduler job.)
     ConfigSpec("plex_max_transcodes", "int", "plex", "plex_max_transcodes", min=0, max=16),
     ConfigSpec("plex_max_sessions", "int", "plex", "plex_max_sessions", min=1, max=64),
-    # Transcode re-encode tuning (E6 S1). The libx264 knobs apply only to the software fallback.
+    # Transcode re-encode tuning. The libx264 knobs apply only to the software fallback.
     # crf 0–51 (x264 range); max_height 0 = source.
     ConfigSpec("plex_transcode_hw", "choice", "plex", "plex_transcode_hw"),
     ConfigSpec("plex_transcode_preset", "choice", "plex", "plex_transcode_preset"),
@@ -210,10 +210,10 @@ SPECS: tuple[ConfigSpec, ...] = (
     ConfigSpec("plex_audio_loudnorm", "bool", "plex", "plex_audio_loudnorm"),
     # NOTE: plex_watch_sync_interval_min is a config.py default read by the watch-sync scheduler job
     # (scheduler.py); it is intentionally NOT a runtime admin knob, so it stays out of SPECS.
-    # --- TMDB enrichment (E5 S2; optional) — the API key is the only secret ---
+    # --- TMDB enrichment (optional) — the API key is the only secret ---
     ConfigSpec("tmdb_api_key", "str", "tmdb", "tmdb_api_key", secret=True),
     ConfigSpec("tmdb_language", "str", "tmdb", "tmdb_language"),
-    # --- Bug reports (E16): the Gitea issue tracker a debug session's issue goes to ---
+    # --- Bug reports: the Gitea issue tracker a debug session's issue goes to ---
     ConfigSpec("issues_gitea_url", "str", "issues", "issues_gitea_url"),
     ConfigSpec("issues_gitea_repo", "str", "issues", "issues_gitea_repo"),
     ConfigSpec("issues_gitea_token", "str", "issues", "issues_gitea_token", secret=True),
@@ -221,7 +221,7 @@ SPECS: tuple[ConfigSpec, ...] = (
     # is a config.py default — both stay out of SPECS, like plex_sync_interval_min.)
 )
 
-# The admin page's sections (E31, the owner's pick): sixteen groups as tabs wrapped to three rows at 133%
+# The admin page's sections (the owner's pick): sixteen groups as tabs wrapped to three rows at 133%
 # zoom, so they are a vertical list under four headings, in this order. Every group of SPECS sits in
 # exactly one section (test-held), so a new group cannot fall off the page.
 SECTIONS: tuple[tuple[str, tuple[str, ...]], ...] = (
@@ -275,7 +275,7 @@ def _coerce(s: ConfigSpec, raw: str):
     if s.type == "int":
         # Bounds are checked on READ, not only in `set_value`. Bounds get ADDED to a spec
         # (download_upload_max_bytes gained a `max` when SEC-20 started enforcing the same number at
-        # the edge; its `min` went 0 → 1 during E7) and env defaults get LOWERED, and either leaves
+        # the edge; its `min` went 0 → 1) and env defaults get LOWERED, and either leaves
         # a stored value that was legal when it was written. Serving it is how one layer ends up
         # acting on a number another layer refuses, with the admin page showing a value outside the
         # range it renders.
@@ -430,7 +430,7 @@ def describe(db: Session) -> dict:
     only whether an override is set and whether an env default exists."""
     groups: dict[str, list[dict]] = {}
     # Every override in one query: a db.get per key was one round trip each (two for a key with no
-    # row, which the session cannot cache), ~40 ms of the page's ~55 on prod (E31 S4d).
+    # row, which the session cannot cache), ~40 ms of the page's ~55 on prod.
     rows = {r.key: r for r in db.scalars(select(SystemConfig))}
     for s in SPECS:
         row = rows.get(s.key)