Add Discord scheduling polls with interactive availability

[nextinfinity]

Summary

Adds Discord-native, date-only scheduling polls, with no external website required.

• /poll opens a creation form for title, inclusive date range, and seven weekday checkboxes (all selected initially).
• Dates accept M/D or MM-DD with year inference, or explicit YYYY-MM-DD. Inferred start dates use today in UTC; inferred end dates are relative to the start, including December–January rollover.
• Members check available dates in a private form and can edit or withdraw their responses. Unchecked dates are unavailable; nonrespondents are not counted as unavailable. Longer polls use multiple checkbox groups in one form.
• The original message displays a live, chronological availability list, highlighting the strongest dates. Details shows member responses with pagination.
• The creator or server admins can finalize a date or cancel. Finalization can optionally create a Discord scheduled event, collecting time, timezone, duration, and voice channel/external location then.
• SQLite persists polls, responses, message IDs, final dates, and event state. Compose mounts a writable persistent volume for the non-root bot.

Implementation notes

• Poll interactions have their own listener so /poll can open a modal without the existing music/timer command listener deferring it first.
• Default Administrator command permissions remain delegatable through Discord. Management controls separately enforce creator/admin access, and all poll interactions check guild and original-channel visibility.
• Message edits are briefly batched and serialized per poll to prevent stale edits overwriting newer totals.
• Event creation has a persisted exclusive claim and best-effort reconciliation against a bot-created event marker before retrying. Permission/API failures preserve the selected date.
• Includes scripts/run-local.sh for a temporary-bot smoke test, with a hidden token prompt, and a detailed manual checklist in docs/scheduling.md.

Intentional v1 limits

• Maximum 25 candidate dates within a 90-day range; creation only in standard server text channels.
• Availability only: no Maybe state, times in voting, reminders, automatic closing, reopening, or automatic winner selection.
• All controls/results are on the original message. No results lookup command or visible poll UUID; deleting the message makes the poll inaccessible in Discord.
• No migration/compatibility logic for the earlier local prototype; its test database was reset. No existing production polling schema exists.

Testing

• ./gradlew --no-daemon check shadowJar passes on JDK 25 and the latest main Gradle wrapper (9.8.1).
• 121 tests pass, including existing music/timer tests and scheduling tests for date inference, weekday filtering, checkbox grouping/defaults, vote replacement/withdrawal, SQLite restart persistence, concurrent voters, stale closed-poll submissions, permissions, serialized updates, and event success/failure/retry handling.
• Creation and response UI were iterated through live testing with a temporary Discord bot. The full event/permissions/restart/mobile smoke-test checklist is documented for further validation.

No bot tokens, test databases, or build artifacts are included.