feat(matrix): support encrypted messages with E2EE

[RussellLuo]

📝 Description

• Add crypto_database_path and crypto_passphrase configuration
• Integrate cryptohelper for decrypting m.room.encrypted events
• Handle both plaintext and encrypted messages in handleMessageEvent
• Enable goolm build tag for libsignal crypto support

🗣️ Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update

🤖 AI Code Generation

• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #1840.

📚 Technical Context (Skip for Docs)

See here for the example of implementing E2EE using maunium.net/go/mautrix.

🧪 Test Environment

• Hardware:
• OS:
• Model/Provider:
• Channels:

📸 Evidence (Optional)

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[yinwm]

LGTM. E2EE support is well-implemented with proper error handling and backward compatibility. No critical issues found.

[RussellLuo]

@yinwm Thanks. make build-all started failing (this job), so I opened a follow-up PR to fix it. The platform-specific details we found later are in the MR: #1975.