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/auth.py

@@ -67,7 +67,7 @@ def has_write_scope(user: User) -> bool:
 def env_allowed_canonicals() -> set[str]:
     """The env bootstrap allow-list, in CANONICAL form — the only shape it may be compared in.
 
-    S3 canonicalised the Invite branch of `is_allowed` and left this one matching the literal string,
+    Canonicalising only the Invite branch of `is_allowed` left this one matching the literal string,
     which is the 2026-09-03 lockout with a different source of truth: an operator who writes
     `first.last@gmail.com` into ALLOWED_EMAILS is refused when Google answers with the undotted
     primary, and a `+tag`ged entry can never match at all, because a Google account cannot contain a
@@ -340,12 +340,12 @@ def _session_user(request: Request, db: Session, user_id: int | None, is_default
     if default_id and default_id not in accounts:
         accounts.append(default_id)
         request.session["account_ids"] = accounts[-MAX_SESSION_ACCOUNTS:]
-    # Attribute this request's log lines to the account (E22). Here, not in the request middleware:
+    # Attribute this request's log lines to the account. Here, not in the request middleware:
     # this is the one place a session becomes an account, and the middleware runs outside both the
     # session middleware and this task's context. `request.state` is shared across middleware layers
     # (it lives on the ASGI scope), so the access-log line can read it back. The user's consented
-    # diagnostics level rides along: their requests are logged — and stored — in that detail (S5).
-    # So does a debug session the request names (E16): only the caller's own open one counts, and a
+    # diagnostics level rides along: their requests are logged — and stored — in that detail.
+    # So does a debug session the request names: only the caller's own open one counts, and a
     # header naming anything else is ignored rather than refused — a stale tab must keep working.
     session = debugsession.from_header(db, user, request.headers.get(debugsession.HEADER))
     level, until = debugsession.level_for(user, session)