Add API endpoint integration tests + library scaffolding

Closes the next-biggest test gap after migration tests. The daemon has
54+ HTTP endpoints, all previously untested at the request/response
level — same shape of blind spot that allowed the v0.1.0:39 migration
bug to ship.

What's new:

- src/lib.rs — exposes the daemon's modules as a library so integration
  tests can import them (`pub mod api;`, etc.). Module source files are
  unchanged; main.rs now imports via `use keysat::...` instead of
  declaring `mod api;` directly. No runtime behaviour change in the
  binary.

- tests/api.rs — 5 integration tests that drive real HTTP requests
  through axum::Router::oneshot against a real SQLite tempfile pool
  (same options as src/db/mod.rs::init):
    1. health_endpoint_returns_200 — framework smoke test
    2. admin_endpoint_rejects_missing_or_wrong_auth — 401 vs 403 paths
    3. admin_creates_product_with_correct_token — full happy path
       (auth → handler → DB insert → audit log → response)
    4. validate_rejects_unsigned_garbage — early parse-fail surfaces
       as `ok: false, reason: "bad_format"` (HTTP still 200)
    5. validate_accepts_well_formed_license — issues a license via
       repo, signs a matching LicensePayload with the daemon's
       actual key, encodes to wire format, validates via the
       endpoint, asserts ok=true plus populated metadata fields

Test count: 9 unit + 4 migrations + 5 API = 18 (was 13).

Cargo.toml dev-deps gain `tower = { version = "0.4", features = ["util"] }`
for ServiceExt::oneshot. The main `tower` dep is feature-minimal because
axum only needs a subset.

Out of scope (explicit follow-ups):

- Purchase happy path (needs a MockPaymentProvider implementing the
  trait; ~250 LOC of mock + ~200 LOC of test).
- Webhook handler with idempotency assertions (same MockPaymentProvider
  dependency).
- Tier-cap enforcement (mechanically simple; small follow-up PR).
- Discount-code atomic reserve race (better as a SQL-layer unit test
  than an HTTP integration test).
- Rate-limiting (interacts with shared state; needs careful isolation).
- Cookie/session auth (already covered in session_layer.rs).

licensing-service/Cargo.toml

@@ -103,6 +103,11 @@ mime_guess = "2"
 # us NamedTempFile so the OS cleans up if a test panics mid-run.
 tempfile = "3"
 
+# tower in main deps is feature-minimal (only what axum needs). Tests use
+# `ServiceExt::oneshot` to drive the router without bringing up an HTTP
+# listener — that lives behind the `util` feature.
+tower = { version = "0.4", features = ["util"] }
+
 [profile.release]
 opt-level = 3
 lto = "thin"