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/tests/test_plex_client.py

@@ -1,4 +1,4 @@
-"""plex/client — the one door to the Plex server (E32 S2: 31% covered).
+"""plex/client — the one door to the Plex server (it was 31% covered).
 
 What matters here is what the callers cannot see: a failure must surface as PlexError (never as an
 empty answer), the paged reads must stop where they should, and the write URIs must be well formed.
@@ -79,7 +79,7 @@ def test_a_section_page_asks_for_guids_and_reports_the_server_total(plex):
     assert params["includeGuids"] == "1" and params["type"] == "1"
     assert params["X-Plex-Container-Start"] == "200" and params["X-Plex-Container-Size"] == "50"
 
-    # Measured on the real server (E32 review): a paged request carries `totalSize` even when the
+    # Measured on the real server: a paged request carries `totalSize` even when the
     # page is empty, past the end, or the section holds nothing — `size: 0, totalSize: 0`.
     plex.routes["/library/sections/7/all"] = {"Metadata": [], "size": 0, "totalSize": 0}
     assert plex.section_items("7", 2) == ([], 0)
@@ -168,7 +168,7 @@ def test_a_collection_is_created_with_a_well_formed_uri(plex):
 
 def test_a_server_without_a_machine_id_refuses_the_write(plex):
     """No machineIdentifier used to become `server:///…` in the write URI — a malformed request
-    Plex answers in its own way. It is refused here, named (E32 S2)."""
+    Plex answers in its own way. It is refused here, named."""
     plex.routes["/identity"] = {}
     plex.routes["/library/collections"] = {"Metadata": [{"ratingKey": "c1"}]}
     with pytest.raises(PlexError, match="machine"):