perf(prisma): add composite indexes for User and Availability tables

[diogeneszilli]

Summary

• Add (email, emailVerified) composite index on User table to optimize login/verification lookups
• Add (scheduleId, date) and (userId, date) composite indexes on Availability table to optimize date range queries
• Migration uses CREATE INDEX CONCURRENTLY for zero-downtime deployment
• Intentionally conservative: only targets tables with low existing index count (User: 4→5, Availability: 3→5) to avoid write amplification on index-heavy tables like Booking (12 indexes)

Test plan

• Verify migration runs cleanly with yarn workspace @calcom/prisma db-migrate
• Verify yarn prisma generate succeeds after schema change
• Run yarn type-check:ci --force to confirm no type regressions

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