Run Analytics on native PostgreSQL (#5)

[AoLiGei1221]

Closes #5

What changed

• replace the Analytics time_bucket() query with PostgreSQL 15 date_bin() using a fixed UTC Monday origin
• preserve the existing API, filters, ordering, and hour/day/week output formats
• add a seven-event PostgreSQL 15.13 fixture covering the ISO week boundary and empty results
• require that fixture in CI

Verification

• pnpm --filter @humanly/backend test:analytics-postgres
• pnpm --filter @humanly/shared build
• pnpm --filter @humanly/backend build
• pnpm test:audit
• built backend contains date_bin and no time_bucket

This PR does not change legacy migrations or the database Compose image.