feat: add Postgres storage adapter

[jamemackson]

Summary

Implements the Postgres storage adapter requested in #222. Enables persisting eval results to PostgreSQL for team-wide visibility and CI/CD durability.

• Add createPostgresStorage() in evalite/postgres-storage implementing the full Evalite.Storage interface
• Support both connection string and individual connection parameters (host, port, database, user, password, ssl, maxConnections)
• Use postgres.js as an optional peer dependency — zero native deps, native JSONB handling
• Register in multi-adapter test suite (conditional on EVALITE_TEST_POSTGRES_URL)
• Add Storage guide to docs with DDL, config examples, and options reference
  Include minor changeset

Design decisions

• No auto-migration — consumers provide their own DDL (documented in the storage guide)
• search_path set at connection level — ensures all connections in the pool resolve unqualified table names correctly
• undefined → null coercion for JSON fields — postgres.js rejects undefined values, so the adapter normalizes them

Test plan

• pnpm run ci passes (build, 160 unit tests, lint, format)
• With EVALITE_TEST_POSTGRES_URL set, all 70 storage tests pass against Postgres alongside SQLite and InMemory
• evalite run persists results to Postgres and evalite serve reads them back in the UI
• Docs preview: Storage guide renders correctly in sidebar under Guides > Storage

Closes #222

[changeset-bot]

🦋 Changeset detected

Latest commit: 11b438e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
evalite	Minor

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