[Multi-year #7] Year-aware stats with year-switcher dropdown

[Mariatta]

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

Scope

Make all stats year-aware. Add a year-switcher dropdown to existing stats pages.

Includes

• Refactor every aggregation function in portal/common.py to accept a conference parameter (about 20 functions)
• Namespace stats cache keys by year (e.g. volunteer_signups_count_2025)
• Update portal/views.py::stats and stats_json to read ?year= query param; default to Conference.get_active()
• Add year-switcher dropdown to:
  • templates/portal/stats.html
  • templates/volunteer/volunteer_stats.html
  • templates/sponsorship/sponsorship_stats.html
  • templates/attendee/attendee_stats.html
• When the selected year predates the portal (no live data exists), render historical_snapshot values with a banner: "Limited data — this conference predates the portal."

Out of scope

• The year-over-year comparison page (Issue #N — extracted to its own page)
• Removing the existing hardcoded HISTORICAL_STATS/PROPOSALS_2025_COUNT constants (deferred to Issue #N when comparison page is extracted)

Related PRs that may conflict

• Added GeoCharts to visualize where the volunteers are from #309 (GeoCharts) — adds a new aggregation function to portal/common.py; rebase plan: add conference parameter to the new function

Dependencies

• [Multi-year #3] Tighten constraints: non-null FKs, VolunteerProfile OneToOne→FK #323 (need non-null FKs to filter aggregations)

Acceptance criteria

• Every aggregation function filters by conference
• Cache keys are namespaced by year
• Year-switcher dropdown works on all 4 stats pages
• ?year=2025 shows live 2025 data; ?year=2023 shows snapshot with banner
• stats.json?year=YYYY returns year-scoped data