[Admin][API] Meter platform-paid AI into the ledger (embeddings/summaries/translation) #44

Follow-up split from #32 (PR #43). The platform-paid env.AI calls are not written to ai_usage, so platform spend is invisible in-app.

Instrument the platform call sites to ledger with provider="platform" + a CF Workers-AI neuron cost estimate (label it an estimate):

• agent-storage.ts embeddings (@cf/baai/bge-base-en-v1.5, ~line 345) and conversation summary (~line 988) — needs the acting/owner userId threaded into the DO storage engine (getStorageEngine).
• routes/instances-translation.ts platform path (has session.uid; the CF .run returns no token usage → estimate tokens from text length).
• Add a CF-neuron estimator to lib/ai-pricing.ts so platform CF rows carry non-zero cost (BYOK-CF stays estimate-free — user pays CF directly).

Once done, /v1/admin/spending platformPaid.metered flips true and the split in aggregateAdminUsage (already provider-aware, PLATFORM_PROVIDER) populates automatically.

Part of the PAGS Admin Portal epic (#27). Depends on #28.