Add db package entrypoints

[Alex-Parejo]

Proposed Changes

• Adds explicit main and types entrypoints to @freelanceflow/db.
• Uses src/index.ts, matching the source-first workspace package style already used in this repo.
• Keeps the fix limited to package metadata.

Proof

node -e "const fs=require('fs'); const p=JSON.parse(fs.readFileSync('packages/db/package.json','utf8')); if (p.main !== 'src/index.ts' || p.types !== 'src/index.ts') process.exit(1); console.log(JSON.stringify({name:p.name,main:p.main,types:p.types}))"
# {"name":"@freelanceflow/db","main":"src/index.ts","types":"src/index.ts"}

git diff --check
# passed

Related Issue

Fixes #4089

/claim #743