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.

frontend/src/i18n/backendIds.contract.test.ts

@@ -6,7 +6,7 @@ import { vocabKey } from "../lib/vocab";
 // up with the raw id as the fallback — `t(\`config.fields.${key}.label\`, key)` — so a missing key never
 // fails loudly: the admin Configuration page showed `maintenance`, `tmdb`, `tmdb_api_key`,
 // `plex_overlay_map`… as bare identifiers, for months, and the i18n parity test stayed green because EN
-// and HU were missing the SAME keys (owner report, E11 S5). The ids live in Python, so the check reads
+// and HU were missing the SAME keys (owner report). The ids live in Python, so the check reads
 // the registries straight from the backend sources:
 //   - `sysconfig.SPECS`      → `config.groups.<group>`, `config.fields.<key>.label` + `.hint`, and for a
 //     "choice" key `config.choices.<key>.<option>` — the options read from its Literal in config.py
@@ -171,7 +171,7 @@ const youtubeReasons = [
   ).matchAll(/"(\w+)"/g),
 ].map((m) => m[1]!);
 
-// E30: the closed English vocabularies the backend WRITES into data, named through `vocab.<group>`
+// The closed English vocabularies the backend WRITES into data, named through `vocab.<group>`
 // (lib/vocab) by their letters-and-digits key. A word the backend adds shows in English until named —
 // so each list is read from its source here: the TMDB crew jobs kept, every auto-tag the tagger can
 // create, and the built-in download profiles of the latest migration that seeds them.