Add necessary DB migration step for localhost instructions

[b8zhong]

Summary

This PR adds an explicit database migration step to the local setup instructions. This is necessary because without running migration - you will get DB errors without doing so.

Changes

Updates the "Running locally" section of the README to include the database migration step:

- pnpm install
- pnpm dev
+ pnpm install
+ pnpm tsx lib/db/migrate.ts && pnpm run build
+ pnpm dev

Why this is necessary

1. On local deployment, the database schema needs to be initialized before authentication can work
2. If not, you get authentication failures on first setup (#450, #731)

Testing Done

• [x] Verified fresh setup works with these instructions
• [x] Confirmed authentication works after following updated steps

Closes #450

[vercel[bot]]

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[b8zhong]

@jeremyphilemon Sorry to bother you but noticed you were active here; would you mind taking a look? Thanks in advance!

[adam-from-meiro]

Thank you

[b8zhong]

@adam-from-meiro Glad you found it helpful :)