test(e24): close the review's coverage gaps, and correct a comment that was untrue

`test_only_a_show_row_pays_for_a_resolution` seeded no watch state, so every rail query
inner-joined its way to zero rows and the loop body — with it the assertion — never ran. It
now seeds a video, a movie and an episode, and asserts all three kinds are present before
the loop, so it cannot go vacuous again.

The Plex half of the surprise rail had no test for either of its promises: every test that
reached `plex_surprise` passed budget="any" against a library with no state rows, so the
duration bounds and the already-started exclusion could both be deleted with the suite
green. The YouTube half has had tests for the same two rules since S1 — the asymmetric half
of one principle.

`plex_item` and `plex_show` — two of the five pinnable kinds — were pinned by no test at
all, leaving the rating_key → internal-id translation and both shelf arms uncovered,
including the self-reference cycle that once 500'd the shelf. And `GET /api/discover/pick`
was exercised only through its domain function, so the route's own query-string translation
could regress silently; the new test excludes all three candidates, which is what proves the
whole list parsed rather than one unsplit string.

`PIN_COLUMNS`/`PIN_KINDS` claimed in a comment to be the one place the pin vocabulary is
spoken and had no readers anywhere: `resolve_ref` re-spelled the column names and the route
re-typed the kind list. Both now drive off the constants.

The `_NO_TS` sentinel was introduced as a crash fix and is not one. The key it replaced,
`(ts is not None, ts)`, cannot raise — tuple comparison finds the first UNEQUAL element via
`==`, so two null keys compare equal and `None < None` is never reached; three independent
runs confirmed it over every permutation. The rationale now says what the sentinel actually
buys, lives once in `plex.rollup`, and the third rail that was still on the older spelling
has been swept to match. Its test is renamed and re-scoped to the mutation it does catch.

Frontend: the reroll's second press asserted `expect.any(Array)`, which cannot tell an
accumulating exclusion window from a replacing one — both that and SEEN_WINDOW = 1 (each
reinstating the measured nine-card cycle) left the suite green. And `onInfo`'s jump type was
untested while `onPlay`'s was pinned twice, so pointing the ⓘ affordance at the player
typechecked and shipped green.

All eight fixes mutation-checked; every mutant died on its intended test.

backend/app/routes/discover.py

@@ -29,7 +29,9 @@ router = APIRouter(prefix="/api/discover", tags=["discover"])
 
 
 class PinIn(BaseModel):
-    kind: Literal["video", "channel", "plex_item", "plex_show", "playlist"]
+    # From `pins.PIN_KINDS`, not a second hand-written list: the vocabulary, the schema columns and
+    # this request contract have to move together, and re-typing them here is how they would not.
+    kind: Literal[*pins.PIN_KINDS]
     # The id the rest of the API speaks for that kind: a video id, a channel id, a Plex rating_key,
     # or a playlist id. `pins.resolve_ref` does the translation to the row's internal key.
     ref: str