Manual adjustments have no effective_date — excluded from weekly/monthly summaries

[timpara]

Problem

The adjustments table stores only created_at. As a result:

• Weekly and monthly summaries ignore manual adjustments.
• Only the all-time total summary reflects them.

This makes it impossible to attribute e.g. on-call comp time to the week/month it was earned.

Proposal

1. Add an effective_date TEXT NOT NULL DEFAULT (date('now')) column to adjustments, with schema migration keyed on schema_meta.schema_version.
2. Update POST/PUT adjustment API to accept effective_date.
3. Roll adjustments into `/api/summary/weekly` and `/api/summary/monthly` bucketed by `effective_date`.
4. Frontend: date picker on the adjustment form, default = today.
5. Tests for bucketing + migration from v1 → v2.

Out of scope

Back-dating existing adjustments (will default to their created_at).