chore: trim production startup logs and add guild join observability

[adrunkhuman]

Goal

Make production logs quieter and more useful around bot startup and server invites.

Current state

Startup currently emits several redundant structured log lines, including decorative banners and step-by-step startup narration. The bot also has no explicit on_guild_join log/action, so inviting the already-running bot to a server is less visible than a restart/on_ready pass.

Scope

• trim decorative/redundant startup logs
• keep high-value startup milestones: environment, DB initialization failure/success, command sync result, ready guild count, permission warnings
• avoid emoji/decorative symbols in production JSON log messages
• add a small on_guild_join log for invite observability
• keep Discord-facing behavior unchanged unless a separate onboarding issue is opened

Non-goals

• no full logging framework rewrite
• no in-channel onboarding message
• no metrics/telemetry backend
• no milestone assignment yet