chore: Analytics cleanup

[isTravis]

Now that the PG analytics migration is live (~19M rows imported, matviews populated, /api/ev endpoint receiving events), this PR removes the leftover Stitch/Redshift plumbing and adds a skeleton loading state to the Impact dashboard.

Stitch removal

• Client sends to one endpoint — plugin.ts now only beacons to /api/ev (removed the dual-write to /api/analytics/track which was Fastly-rewritten to an AWS Lambda)
• Removed isProd from all client analytics payloads (usePageOnce.ts, Download.tsx) — the server was already stripping it via DROPPED_FIELDS
• Removed /api/analytics/track from middleware allowlists (readOnlyMiddleware.ts, communityBanGuard.ts) — that path never reached Express
• Removed STITCH_WEBHOOK_URL and AM_REDSHIFT_* env vars from envSchema.ts, .env.test, ENV.md, env.example
• Updated README.md — data flow diagram, endpoint paths, removed one-time migration section, updated file map

Skeleton loading

• Replaced the <Spinner> loading state on the Impact dashboard with a skeleton that mirrors the actual layout (stat cards, chart area, table grids)
• Pulsing animation, dark mode support
• Removed the temporary SHOW_MIGRATION_BANNER constant and <Callout> block

Kept

• tools/migrateRedshift.ts — retained for historical reference (consistent with existing pattern of keeping past migration tools). Updated header docs.
• DROPPED_FIELDS in server/analytics/api.ts — keeps accepting isProd, communityName, etc. from cached clients gracefully