[Feature]: add structured application logger

[viiccwen]

Summary

Add a shared JSON Lines application logger with consistent event fields and default redaction or hashing for sensitive values. Migrate the notification service as the initial structured-logging slice while preserving stderr and Docker log compatibility.

Related Issue

Fixes #90

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Configuration or deployment
• Tests

Changes

• Add a thread-safe structured logger using the project's Taipei timezone and stderr by default.
• Add hashing and redaction policies for account IDs, emails, tokens, exceptions, and user-provided content.
• Migrate Discord notification delivery logs and cover formatting, privacy, failure paths, timezone handling, and JSON edge cases.

Testing

• Local tests or checks pass: make test (257 passed)
• Manual verification completed: make precommit and git diff --check
• Not tested; reason:

Impact Checklist

• Discord bot behavior checked, if affected
• MCP server behavior checked, if affected
• Webhook behavior checked, if affected
• Docker or compose changes checked, if affected
• Environment variables documented, if changed
• Logs do not expose secrets or sensitive data

Notes for Reviewers

Issue #90 intentionally requests a small initial migration slice. This PR migrates the complete notifications module; remaining application logs can move incrementally under the broader observability work in #89.