fix: embed IANA timezone database to fix startup failure on Windows

[octo-patch]

Fixes #1398

Problem

On platforms without a system-level timezone database (notably Windows), time.LoadLocation() fails with unknown time zone Asia/Shanghai, causing the server to exit at startup:

Failed to initialize application: invalid timezone "Asia/Shanghai": unknown time zone Asia/Shanghai

Go's time.LoadLocation relies on the OS timezone data (/usr/share/zoneinfo on Linux/macOS), which is not available on Windows.

Solution

Add a blank import of time/tzdata in backend/cmd/server/main.go. This embeds the complete IANA timezone database directly into the compiled binary (~450 KB overhead), making all time.LoadLocation calls work on every platform without requiring OS timezone data.

import _ "time/tzdata" // embed IANA timezone database for platforms without system tzdata (e.g. Windows)

Testing

• Root cause confirmed by tracing timezone.Init() in internal/pkg/timezone/timezone.go through internal/repository/ent.go
• This is the standard, well-documented Go solution: https://pkg.go.dev/time/tzdata