Fix email case normalization across all partner records

Partner emails are stored with inconsistent casing (e.g., 'Alice@Example.COM' vs 'alice@example.com'), which breaks duplicate detection and causes the same organization to appear multiple times in the system.

Two parts to this fix:

Part 1 — Migration: Write a database migration (using Drizzle migrate) that lowercases all existing partner.email values in place. The migration must be idempotent (safe to run twice).

Part 2 — Validation: Add normalization at the input layer so new and updated partner records always save emails in lowercase:

• In the zod schema for partner creation/update, add .toLowerCase() transform on the email field
• Add a PostgreSQL check constraint as a safety net: CHECK (email = lower(email))

Write tests that confirm:

1. Submitting 'User@EXAMPLE.com' saves as 'user@example.com'
2. A uniqueness check correctly identifies 'User@EXAMPLE.com' as a duplicate of an existing 'user@example.com' record
3. The migration does not error on already-lowercase emails