Poll the store every 30s while the Friday release lands

[mspinola]

Why

The store poller checked every 5 minutes, always. After a Friday release reached the server, it could therefore sit there for up to 5 minutes before the rebuild and the weekly email started. With the rebuild now under a minute (cotmetrics 0.14.2 and 0.14.3), that wait is the largest delay left that this box controls.

What

• The poller checks every 30s on Fridays 15:25 to 16:30 ET. That covers the CFTC's 15:30 release, the Windows producer's Friday poll (every 2 minutes from 15:25 for 45 minutes), and sync time.
• seconds_until_next_poll also caps the sleep at the window's start, so a 5-minute sleep begun at 15:24 does not carry the first tick to 15:29.
• The always-on 5-minute baseline stays. The replica can move at times no schedule predicts, so that part must never become a window. The window only tightens it, and a tick is two small JSON reads.
• The window is Fridays only because the producer polls only on Fridays. A holiday-week release reaches this box with the next 08:10 catch-up regardless.
• The startup log line now states both cadences.

Tests

tests/test_store_poll_interval.py (11 tests) covers the window edges, other days, a sleep capped at the window's start, and the window staying in Eastern time when now arrives in UTC, summer and winter. Full suite: 897 passed. ruff check src tests clean.

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