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/routes/channels.py

@@ -67,8 +67,8 @@ def list_channels(user: User = Depends(current_user), db: Session = Depends(get_
 
     # Per-channel aggregates over the shared catalog (channel_aggregates), computed on read rather than
     # denormalised: an Index Only Scan over a ~20 MB index, where it used to read every video's heap page
-    # (681 MB on prod, a 2–6 s first open once something else had pushed it out of memory, E31 S4d).
-    # (The Normal/Shorts/Live breakdown moved to the channel-detail page in R14, so it is no longer
+    # (681 MB on prod, a 2–6 s first open once something else had pushed it out of memory).
+    # (The Normal/Shorts/Live breakdown moved to the channel-detail page, so it is no longer
     # computed here — the manager table dropped its N/S/L column.)
     channel_ids = [c.id for _, c in rows]
     agg: dict[str, dict] = {}
@@ -276,7 +276,7 @@ def _channel_detail_dict(
         "explored": explored,
         "blocked": blocked,
         "stored_videos": stored,
-        # Per-type breakdown (Normal / Shorts / Live) — R14 moved it off the manager table onto the
+        # Per-type breakdown (Normal / Shorts / Live) — moved off the manager table onto the
         # channel page. Same aggregation the manager list uses (count_normal = stored − shorts − live).
         "count_normal": count_normal,
         "count_short": count_short,
@@ -349,7 +349,7 @@ def channel_detail(
     )
     blocked = _is_blocked(db, user, channel_id)
     # Stored total + the Normal/Shorts/Live split in one grouped pass (same filters as the manager
-    # list), so the channel page can show the per-type breakdown R14 moved off the table.
+    # list), so the channel page can show the per-type breakdown that moved off the table.
     n_all, n_short, n_live = db.execute(
         select(
             func.count(Video.id),