Add start-time gating with timezone-aware queue display

[Copilot]

The queue must hold users until a configured start moment, respecting timezone, and surface that schedule in the waiting page.

• Config & validation

  • Added startTime (RFC3339) and startTimeZone (IANA TZ) with parsing/validation and timezone-normalized comparisons.
  • Documented interaction: RFC3339 offset respected, then converted to provided zone.

• Request handling

  • Gate traffic before start time using the configured zone; bypass once reached.
  • Queue page data now includes start time, seconds-until-start, and reach flag.

• UI templates

  • Custom and fallback queue pages render the start time and a live countdown (immediate first tick).

• Tests

  • Added coverage for invalid start time/zone, blocking before start, and allowing after start.

Example configuration:

- traefik.http.middlewares.qm.plugin.queuemanager.startTime=2026-02-01T10:00:00Z
- traefik.http.middlewares.qm.plugin.queuemanager.startTimeZone=Europe/Paris

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