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/tsconfig.node.json

@@ -1,7 +1,7 @@
 {
   // The build-tool configs run in Node, not the browser: they need node types and must NOT be
   // checked against the app's DOM lib. Split out because tsconfig.json's `include` is ["src"],
-  // so until now nothing type-checked these files at all (R2 / C-4.5). `npm run typecheck`
+  // so until now nothing type-checked these files at all. `npm run typecheck`
   // runs this project alongside src and e2e.
   "compilerOptions": {
     // composite: required to be the target of tsconfig.json's `references` (which is what makes
@@ -23,20 +23,20 @@
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "noFallthroughCasesInSwitch": true,
-    // E32: an index read is `T | undefined` here, as it really is (0 errors when switched on).
+    // An index read is `T | undefined` here, as it really is (0 errors when switched on).
     "noUncheckedIndexedAccess": true,
     "resolveJsonModule": true,
     // The build-tool configs include a hand-written .js file (postcss; tailwind 4 needs no config
-    // file, and the stale `tailwind.config.js` entry pointed at nothing until E32). checkJs runs
+    // file, and the stale `tailwind.config.js` entry pointed at nothing). checkJs runs
     // the type checker over it via its `@type` JSDoc, so a bad option is caught by the gate instead
-    // of silently mis-building CSS. Was the last hole in C-4.5.
+    // of silently mis-building CSS. Was the last unchecked file.
     "allowJs": true,
     "checkJs": true
   },
-  // reactPlugin.ts is the ONE React transform both configs import (E27 S1) — it is build tooling,
+  // reactPlugin.ts is the ONE React transform both configs import — it is build tooling,
   // not app source, so it belongs here and not under tsconfig.json's ["src"]. A file imported by a
   // listed one but absent from `include` is TS6307, not a silent pass. playwright.config.ts is
-  // here as well as in e2e/tsconfig.json because eslint's typed rules (E32) find a file's project
+  // here as well as in e2e/tsconfig.json because eslint's typed rules find a file's project
   // through tsconfig.json's `references`, which name this project and not the e2e one.
   "include": [
     "vite.config.ts",