docs: plan for a self-serve ad network

Adds docs/self-serve-ad-network.md, a component-by-component plan for
owning the demand side of Freebuff's own ad slots.

Scope: first-party inventory only, with the ad-request API shaped for
later multi-publisher expansion, and the existing providers kept as
backfill below our own demand.

Covers the placement registry, advertiser console, creative policy, ad
server pipeline, pricing and allocation, mediation floors, the user
credit loop, measurement, invalid traffic, admin tooling, a data model
sketch, the open decisions with recommendations, and phasing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015w3TQVmdCS1SeSCSXsEAzX

Author: claude