[Multi-year #5] Remove auto-create signal; make volunteer application explicit per year

[Mariatta]

Part of the Multi-Year Conferences refactor. See docs/architecture/multi-year-conferences.md for full context.

Scope

Stop auto-creating VolunteerProfile on user signup. Make "Apply to volunteer for PyLadiesCon YYYY" an explicit per-year action.

Includes

• Remove the part of volunteer_profile_signal that auto-creates VolunteerProfile on user signup. Email notification and approval-status logic stay.
• Sign-up creates only PortalProfile.
• Update the "Apply to volunteer" view:
  • Creates a new VolunteerProfile tied to Conference.get_active()
  • Guards the action with Conference.volunteer_application_open
• Update volunteer list/detail/edit views to filter by active conference
• Update related URL routes if needed

Out of scope

• Pre-filling the form from prior year (Issue #N — Returning Volunteer flow)

Related PRs that may conflict

• feat: [WIP] Automate GDrive Access for approved volunteers #114 (WIP — GDrive automation hooks into volunteer/signals.py)

Dependencies

• [Multi-year #3] Tighten constraints: non-null FKs, VolunteerProfile OneToOne→FK #323 (must be able to create multiple profiles per user)

Acceptance criteria

• No VolunteerProfile is auto-created on signup
• "Apply to volunteer" view creates a new profile tied to active conference
• If volunteer_application_open=False, the apply action is blocked with a clear message
• Existing users with a 2025 profile see only 2025 data until they apply for 2026