fix doubled "waiting for manual poll" log on every poll cycle

[ajslater]

Summary

• The poller's main loop iterates all libraries twice per cycle whenever any library is due — first via _get_min_timeout to find the soonest due library, then via _poll_due_libraries to actually poll them.
• Both call _get_poll_timeout, and that helper emits an INFO line Library X waiting for manual poll. as a side effect for every poll=False library it sees. So the line shows up twice every hour for the custom-covers library.
• Folded the two passes into _poll_due_and_get_next_timeout, which polls each due library inline and folds its post-poll poll_every into the running min. Each library is now inspected exactly once per cycle.

Production log evidence (note the duplicated lines):

2026-05-05 14:36:33 | DEBUG    | Move Folders: ?/3
2026-05-05 14:36:33 | INFO     | Polled library /comics in a moment.
2026-05-05 14:36:33 | INFO     | Library /config/custom-covers waiting for manual poll.
2026-05-05 14:36:33 | DEBUG    | Read tags from Comics /comics: 0/0
2026-05-05 14:36:33 | DEBUG    | Library /comics update in progress. Not polling.
2026-05-05 14:36:33 | INFO     | Library /config/custom-covers waiting for manual poll.

Test plan

• make fix and make lint (the unrelated remark error on . is preexisting on develop)
• pytest tests/ — 35 passed
• Sanity-check a running instance: confirm the custom-covers waiting for manual poll INFO line appears once per polling cycle, not twice
• Confirm scheduled polling still fires at the configured interval (no infinite waits when only library was the one just polled — the new code folds the post-poll poll_every into the running min so the wait isn't None)

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