feat: accept smtp_username and smtp_password as separate config fields

[rubenhensen]

Summary

• Replaces smtp_credentials (array) with separate smtp_username and smtp_password config fields
• Enables clean injection via ROCKET_SMTP_USERNAME and ROCKET_SMTP_PASSWORD env vars, each referencing a single Kubernetes secretKeyRef
• Updates config structs, accessor methods, email sending logic, and example config files

Closes #56

Test plan

• Verify SMTP sending works with smtp_username / smtp_password set in config
• Verify SMTP sending works without credentials (unauthenticated)
• Verify env var injection via ROCKET_SMTP_USERNAME / ROCKET_SMTP_PASSWORD