feat: enforce free tier limits (3 videos/batch, 720p max)
- config.py: add BATCHTUBE_PLAN env var, FREE_TIER_MAX_VIDEOS=3,
FREE_TIER_MAX_RESOLUTION=720, PLAN_LIMITS map for all tiers
- api_server.py: cap resolution to 720p on free plan, reject CSV
batches > 3 videos with 403 + upgrade prompt
- worker.py: truncate resolved video list to max_videos on free plan,
emit tier_limit SSE event with upgrade message
Pricing tiers (2026-03-07 approved):
Free: 3 videos/batch, 720p | Creator: $19/mo | Pro: $39/mo
Studio: $79/mo | Lifetime: $197