Per-channel billing and encoding usage stats

cp-broadcaster now publishes detailed per-channel encoding and bandwidth usage events, which feed the platform billing pipeline directly.

• Encoding statistics are emitted at a regular interval covering active encode minutes, resolution ladder, and output bitrate.
• Billing stats publish honours channel start/stop boundaries so partial-hour usage is accurate.
• A toggle was added to pause billing-stats publish during incident mode to avoid double-charging on retries.
• SKU validation is run on each publish so channels only emit usage for entitlements they actually have.
• The same stats feed the channel-analytics surfaces so customers see consistent numbers between billing and dashboard.