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/audit.py

@@ -27,7 +27,7 @@ class AuditAction:
     USER_SUSPEND = "user_suspend"
     USER_UNSUSPEND = "user_unsuspend"
     USER_DELETE = "user_delete"
-    # E9.5 S5: an admin let a stuck registrant in, or re-sent their confirmation link. Both are
+    # An admin let a stuck registrant in, or re-sent their confirmation link. Both are
     # recorded because both are the admin acting ON someone's access, not merely reading a list.
     USER_ACTIVATE = "user_activate"
     USER_VERIFY_RESEND = "user_verify_resend"
@@ -36,7 +36,7 @@ class AuditAction:
     INVITE_DENY = "invite_deny"
     INVITE_ADD = "invite_add"
     INVITE_DELETE = "invite_delete"
-    # E9.5 S6: the admin chose to tell someone their address is whitelisted. Adding the address is
+    # The admin chose to tell someone their address is whitelisted. Adding the address is
     # silent, so this is the act that actually reaches a person — and the one worth a record.
     INVITE_SEND = "invite_send"
     DISCOVERY_PURGE = "discovery_purge"
@@ -55,7 +55,7 @@ class AuditAction:
     DOWNLOAD_CREDENTIAL_DELETE = "download_credential_delete"
     # The log itself was cleared (a single row survives the clear as tamper-evidence)
     AUDIT_CLEAR = "audit_clear"
-    # E16: a user sent a debug session's issue to the tracker — the one act that takes their log
+    # A user sent a debug session's issue to the tracker — the one act that takes their log
     # off this instance, so it is recorded (target = the session, summary = the issue's URL).
     ISSUE_CREATE = "issue_create"