feat: multi-user isolation and graceful shutdown

- Add chat_id to events, settings, and days_off for per-user data
- Add proper graceful shutdown (signal handling, WaitGroup)
- Separate polling and webhook modes with goroutine management
- Add schema migration from single-user to multi-user schema
- Refactor handlers with shared actionFunc pattern (msg + callback)
- Fix schema path for Docker deployment (/app/db/schema.sql)
- Remove emoji from output text for cleaner formatting

Dockerfile

@@ -19,7 +19,7 @@ RUN apk add --no-cache ca-certificates tzdata
 WORKDIR /data
 
 COPY --from=builder /usr/local/bin/worktime-bot /usr/local/bin/worktime-bot
-COPY --from=builder /src/db/schema.sql ./db/schema.sql
+COPY db/schema.sql /app/db/schema.sql
 VOLUME ["/data"]
 
 CMD ["worktime-bot"]