feat(express-context): add shared BillingClient abstraction

[pyramation]

Summary

Extracts billing operations (checkQuota, recordUsage, logInference) into a shared BillingClient in @constructive-io/express-context, eliminating duplication between agentic-server and graphile-llm.

Before: Both packages implemented identical SQL calls for billing quota checks and usage recording independently.

After: Single source of truth via ctx.useBilling():

// In any Express handler or Graphile plugin:
const billing = await ctx.useBilling();
if (billing) {
  const allowed = await billing.checkQuota('text-embedding-3-small');
  // ... do work ...
  await billing.recordUsage('text-embedding-3-small', tokenCount, metadata);
  await billing.logInference({ model, inputTokens, outputTokens, ... });
}

Key design decisions:

• useBilling() is lazy + cached per request — only resolves billing/inferenceLog loaders on first call, returns same instance on subsequent calls
• Returns null if billing_module not provisioned or no entity_id in JWT (graceful no-op)
• graphile-llm adapts Graphile's 2-arg withPgClient(pgSettings, fn) to the 1-arg pattern via billingClientFromCtx() helper
• agentic-server/src/billing.ts deleted entirely — router calls ctx.useBilling() directly

Relates to constructive-io/constructive-planning#1054

Link to Devin session: https://app.devin.ai/sessions/4a9f098c74fb4cb6a9b6868fcff321db
Requested by: @pyramation

[devin-ai-integration]

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