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/plex/people.py

@@ -160,7 +160,7 @@ def people_by_credits(
     return [r[0] for r in rows]
 
 
-# --- Portraits (E30 S6) ---------------------------------------------------------------------------
+# --- Portraits ---------------------------------------------------------------------------
 # A Plex CDN headshot is a full-size original (~170 KB on average, measured). Plex's photo transcoder
 # fits it into this box (~13 KB), which covers the largest place a portrait is drawn (the person page's
 # 6rem avatar) at 2x density and a raised text size. The box is part of the cache key, like a poster's:
@@ -226,7 +226,7 @@ _DEAD = frozenset({404, 410})
 
 
 def warm_portraits(db: Session, limit: int) -> dict:
-    """Cache up to `limit` not-yet-cached headshots, the most-credited people first (E30 S6), so a
+    """Cache up to `limit` not-yet-cached headshots, the most-credited people first, so a
     person's face comes from local disk. Needs Plex (the resize) and the Plex module on; without them
     nothing is prefetched — full-size originals for every person would cost gigabytes. A headshot Plex
     answers "not found" for is marked and left for PORTRAIT_RETRY; Plex failing in a row ends the run."""