P5 exit: usage metering, credit billing, promo codes, 3-step wizard

- LlmEvent::Usage across all three providers (Scripted deterministic
  word-count accounting; Anthropic message_start/delta usage; OpenAI-compat
  stream_options include_usage)
- tc-billing: ceil(tokens/1000) min 1 credit; lots drain oldest-first under
  FOR UPDATE; balance clamps at zero while the usage ledger records the
  full obligation; promo codes redeem exactly once via CAS (migration 0006)
- Runtime charges every completed run (billing failure never fails a run);
  proven: 1 token in + 3 out -> 1 credit deducted
- API: GET /api/team/usage, POST /api/credits/redeem (409 on reuse, audited)
- Credits page: balance, 7-day usage meter with runway estimate, PromoRedeem
- /claws/new is the full §9 wizard: ?step=identity|access|slack deep-linked
  progress, accent swatches + name randomizer, access toggles, optional
  Slack step, explicit review-and-confirm (creation = live agent), animated
  provisioning state -> straight into chat
- E2E: chat decrements the visible balance and fills the usage meter;
  WELCOME500 adds exactly 500 once then refuses; wizard round trip

140 Rust + 63 frontend tests + 23 Playwright journeys.

Co-Authored-By: Claude Fable 5 <[email protected]>

crates/tc-billing/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "tc-billing"
+version = "0.1.0"
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+publish.workspace = true
+
+[dependencies]
+sqlx = { workspace = true }
+tc-domain = { path = "../tc-domain" }
+thiserror = { workspace = true }
+uuid = { workspace = true }
+
+[dev-dependencies]
+tc-db = { path = "../tc-db" }
+tc-testkit = { path = "../tc-testkit" }
+time = { workspace = true }
+tokio = { workspace = true }
+
+[lints]
+workspace = true