Agent connections overhaul — all-day events restored, stricter validation, pricing copy corrected

A broad overhaul of AI agent connections (API and MCP). Behavior now matches the connection guide exactly.

• Fixed all-day event input failing with an error
• Dates that don't exist (like February 31) were silently accepted — they now return a clear error
• Unknown fields and invalid status values now get an explicit error instead of being ignored
• 403 responses include a machine-readable code, and the connection guide now matches actual behavior exactly
• Fixed agent-created categories occasionally duplicating an existing name
• Agent API guide — usage limits are now described in the unit actually enforced (per token)
• Plus a broad polish of internal checks and safeguards, including backup restore and calendar integration