Adds an integrations app module covering the providers from the tier 1/2
plan: Calendly, Cal.com, Google Sheets, Google Postmaster, Microsoft SNDS,
DMARC ingestion, and Cloudflare/GoDaddy/Namecheap DNS. One unified
migration provisions integration_connections, dmarc_reports + record
rows, postmaster_snapshots, dns_verifications, and meeting_bookings.
The service exposes a generic CRUD surface for connection state with
per-provider files for parsing (calendly.go, dmarc.go), HTTP clients
(cloudflare.go, postmaster.go, google_sheets.go), and DNS verification
(dns.go). Inbound webhook routes use per-org URL-embedded secrets so
Calendly/Cal.com/DMARC providers post directly without Warmbly auth.
DNS verifier resolves SPF/DKIM/DMARC + tracking CNAME and surfaces
fixes when a record is missing.
