feat: WoT-aware adaptive PoW policy engine (NIP-02 + NIP-13) #778

Is your feature request related to a problem? Please describe.

nostream now has a Web of Trust graph (#626, ) exposing a distance-lookup API, and a relay-load-aware adaptive PoW pipeline (#755,) that scales required difficulty with observed event rate. The two systems don't talk to each other yet: every pubkey faces the same computed difficulty regardless of how trusted it is, so a well-connected community member and a completely unknown pubkey are treated identically under load.

Describe the solution you'd like

• Wire the WoT graph's distance-lookup API into the adaptive PoW pipeline so the two systems work
  together: high-trust users (low WoT distance) get reduced PoW requirements or bypass PoW entirely, while unknown pubkeys face the full computed difficulty
• A well-connected community member should be able to post instantly under load, while a random pubkey with no social graph presence has to prove they're not a bot
• Configurable policy operators decide the WoT distance thresholds and the corresponding PoW reduction at each threshold
• Tests covering the full flow: graph construction, distance calculation, adaptive PoW calculation, and the combined policy engine deciding the final effective difficulty

Additional context

Depends on the WoT graph's distance-lookup API (#626) and the adaptive PoW difficulty calculation (#755), both already merged. This is a new policy layer combining the two no existing precedent for it in the codebase.