feat(admin): context-aware analytics CSV export

[sentry-junior]

Fixes the empty 2025 ready-video export and makes analytics export useful for both participation retros and year project harvest.

One button, context-aware

• /admin/analytics → Export year metrics CSV (hackweek-year-metrics.csv)
  • one row per year: voters, votes, projects, ideas, participants, ready videos, categories, awards
• /admin/analytics?year=YYYY → Export {year} projects CSV
  • one row per active project/idea, ranked by total votes
  • description, members, awards, category votes
  • optional ready-video fields (has_ready_video, id/url/filename/duration) when present

Why

• ready-video-only export left 2025 (and other pre-R2 years) with headers and no rows
• button still showed on those years → dead end
• retros need multi-year participation metrics; ceremony/supercut still need year project rows

Also

• surfaces Ideas on year metric cards
• no legacy Stream/Firebase video rehydration in this PR

Intended use cases: year-over-year participation/engagement, awards ceremony prep, supercut seed when ready videos exist.

Requested by ckj.

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