feat(allocation): implement resource allocation feature

- Add AllocationController with CRUD + bulk endpoints
- Add AllocationValidationService for capacity/estimate validation
- Add AllocationMatrixService for optimized matrix queries
- Add AllocationPolicy for authorization
- Add AllocationResource for API responses
- Add frontend allocationService and matrix UI
- Add E2E tests for allocation matrix (20 tests)
- Add unit tests for validation service and policies
- Fix month format conversion (YYYY-MM to YYYY-MM-01)

openspec/changes/capacity-expert-mode/decision-log.md

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+# Decision Log: capacity-expert-mode
+
+## 2026-02-24 — Timezone & Accessibility Fixes
+
+### Issue
+User reported that weekend shading in Expert Mode was misaligned (showing on 5th/6th instead of 4th/5th for April 2026). Additionally, the shading was too subtle for colorblind users, and cells were not prefilled with `O`/`H` for weekends/holidays.
+
+### Root Cause
+Browser `new Date('YYYY-MM-DD')` interprets dates in local timezone, causing dates to shift for users west of UTC. A user in UTC-6 sees `2026-04-04` as `2026-04-03T18:00`, which reports as Friday instead of Saturday.
+
+### Decisions Made
+
+| # | Decision | Rationale |
+|---|----------|-----------|
+| D8 | Use Eastern Time (America/New_York) as canonical timezone for weekend/holiday detection | Ensures consistent business-day alignment; future configurable |
+| D9 | Use high-contrast styling for weekends/holidays (solid backgrounds + borders) | Accessibility for colorblind users |
+| D10 | Prefill weekends with `O`, holidays with `H` on grid load | Matches user expectations; reduces manual entry |
+| D11 | Frontend and backend must sync when seeding default values | Prevents divergent behavior on refresh |
+
+### Implementation Notes
+- Frontend: Parse dates as `new Date(dateStr + 'T00:00:00')` to avoid timezone drift
+- Backend: Use Carbon with `America/New_York` timezone
+- Both sides must implement identical prefill logic
+
+### Future Considerations
+- Make timezone configurable per-team or per-user (v2)
+- Extract prefill rules to shared configuration
+
+---
+
+## Earlier Decisions
+
+See `design.md` sections D1-D7 for original design decisions.