feat(intake): token generator and shape check

Phase 8a Task 2. 24 bytes of CSPRNG entropy, base64url-encoded to
exactly 32 URL-safe characters. isIntakeToken matches that exact
shape so the public route can fail fast on bogus input without
hitting the database or charging the rate limiter.

src/lib/intake/token.ts

@@ -0,0 +1,24 @@
+import { randomBytes } from "node:crypto";
+
+/**
+ * 24 bytes of entropy, base64url encoded → 32 URL-safe characters.
+ *
+ * The token is the link's whole authentication. It must be unguessable and
+ * must not be derived from anything the public can see (no project id, no
+ * timestamp, no hash of either). `crypto.randomBytes` satisfies both: it is
+ * CSPRNG output, and the rest of the system treats the result as opaque.
+ *
+ * 24 bytes is 192 bits of entropy. Brute-forcing one token against the
+ * `(token)` unique index is wasted effort at any realistic rate limit.
+ */
+export function generateIntakeToken(): string {
+  return randomBytes(24).toString("base64url");
+}
+
+const TOKEN_PATTERN = /^[A-Za-z0-9_-]{32}$/;
+
+/** True when `value` is the shape the generator produces. Used by the
+ *  public route to reject obviously-bogus input early. */
+export function isIntakeToken(value: unknown): value is string {
+  return typeof value === "string" && TOKEN_PATTERN.test(value);
+}