feat: add video support for intercom lines (WHIP/WHEP, H264) #341

Tracking issue for PR #315, which adds per-line video: WHIP publish and WHEP consume of H264/VP8 over SMB, video-source pinning with SSRC whitelisting, and the supporting session/line model fields (videoEnabled, hasVideo, isWhepReceiver, whepSourceSessionId) with pin reconciliation when a publisher leaves.

This is the backend half of a paired backend+frontend feature. The frontend counterpart is Eyevinn/intercom-frontend#684 (companion tracking issue to follow).

Filed to get PR #315 onto board #48 and into review, after it sat unreviewed for two days.