feat: add Drone CI pipelines, Jest unit tests, and Prettier check

Add Drone CI with server build/test and format check pipelines.
Add unit tests for password utils and recurrenceExpander.
Add check_format script, fix Jest config to ignore dist/,
remove dead CaldavService.test.ts, apply Prettier formatting.

CLAUDE.md

@@ -14,6 +14,7 @@ This is a fullstack TypeScript monorepo with npm workspaces.
 ```bash
 npm install          # Install all dependencies for all workspaces
 npm run format       # Format all TypeScript files with Prettier
+npm run check_format # Check formatting without modifying files (used in CI)
 ```
 
 ### Client (apps/client) - Expo React Native app
@@ -36,6 +37,7 @@ npm run build -w @calchat/shared        # Compile shared types to dist/
 npm run dev -w @calchat/server          # Build shared + start dev server with hot reload (tsx watch)
 npm run build -w @calchat/server        # Build shared + compile TypeScript
 npm run start -w @calchat/server        # Run compiled server (port 3000)
+npm run test -w @calchat/server         # Run Jest unit tests
 ```
 
 ## Technology Stack
@@ -58,7 +60,9 @@ npm run start -w @calchat/server        # Run compiled server (port 3000)
 | | react-native-logs | Client-side logging |
 | | tsdav | CalDAV client library |
 | | ical.js | iCalendar parsing/generation |
+| Testing | Jest / ts-jest | Server unit tests |
 | Deployment | Docker | Server containerization (multi-stage build) |
+| | Drone CI | CI/CD pipelines (build, test, format check) |
 | Planned | iCalendar | Event export/import |
 
 ## Architecture
@@ -694,6 +698,21 @@ This uses the `preview` profile from `eas.json` which builds an APK with:
 - Package name: `com.gilmour109.calchat`
 - EAS Project ID: `b722dde6-7d89-48ff-9095-e007e7c7da87`
 
+## CI/CD (Drone)
+
+The project uses Drone CI (`.drone.yml`) with two pipelines:
+
+1. **`server_build_and_test`**: Builds the server (`npm ci` + `npm run build`) and runs Jest tests (`npm run test`)
+2. **`check_for_formatting`**: Checks Prettier formatting across all workspaces (`npm run check_format`)
+
+## Testing
+
+Server uses Jest with ts-jest for unit testing. Config in `apps/server/jest.config.js` ignores `/node_modules/` and `/dist/`.
+
+**Existing tests:**
+- `src/utils/password.test.ts` - Tests for bcrypt hash() and compare()
+- `src/utils/recurrenceExpander.test.ts` - Tests for expandRecurringEvents() (non-recurring, weekly/daily/UNTIL recurrence, EXDATE filtering, RRULE: prefix stripping, invalid RRULE fallback, multi-day events, sorting)
+
 ## Documentation
 
 Detailed architecture diagrams are in `docs/`: