feat: full backend (untested)

PLAN.md

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+# Common Availability - Implementation Plan
+
+## Project Overview
+ICS-based calendar availability coordination system. Company members submit ICS URLs, and the system continuously syncs and calculates common availability for scheduling meetings.
+
+## Architecture
+
+### Backend (FastAPI + PostgreSQL)
+- **Models**: `Participant` (id, name, email, ics_url) and `BusyBlock` (participant_id, start_time, end_time)
+- **Services**: ICS fetching/parsing via `icalendar` library, availability calculation
+- **API Endpoints**:
+  - `POST /api/participants` - Add participant with ICS URL (auto-syncs)
+  - `GET /api/participants` - List all participants
+  - `DELETE /api/participants/{id}` - Remove participant
+  - `POST /api/availability` - Calculate availability for given participants
+  - `POST /api/sync` - Sync all calendars
+  - `POST /api/sync/{id}` - Sync specific participant
+
+### Frontend (React + TypeScript)
+- Existing UI preserved (React, Vite, shadcn-ui)
+- API client in `src/api/client.ts`
+- Real-time availability from backend (replaces mock data)
+
+## Current State
+Implementation complete and tested. All systems operational.
+
+## How to Run
+```bash
+just fresh     # Build, start containers, run migrations
+just up        # Start services
+just logs      # View logs
+just migrate   # Run Alembic migrations
+just sync-calendars  # Sync all ICS feeds
+```
+
+Access:
+- Frontend: http://localhost:5173
+- Backend API: http://localhost:8000
+- API docs: http://localhost:8000/docs
+
+## Test Flow
+1. Add participant with ICS URL (e.g., https://user.fm/freebusy/v1-.../Calendar.ics)
+2. Select participants in Schedule tab
+3. View availability heatmap (green = all free, yellow = partial, red = busy)
+4. Click sync button to refresh calendars
+
+## Verified Working
+- [x] ICS parsing and sync (321 busy blocks from sample ICS)
+- [x] Availability calculation
+- [x] Participant CRUD operations
+- [x] Frontend integration
+
+## Next Steps
+- [ ] Add background scheduler for periodic ICS sync
+- [ ] Add calendar revocation mechanism