fix(security): close SSRF, credential-leak, and auth bypass findings in 0.8.4

Audit of 0.8.4 found the repo's own gates green, so these came from manual
review; each is covered by a new regression test.

security/fetch.ts
- safeFetch re-attached Authorization/Cookie on a same-origin redirect that
  followed a cross-origin hop (a -> b -> b), handing credentials to the second
  host. Compare against the origin the caller trusted, not the previous hop.
- The private-network guard resolved the host, approved it, then let fetch
  resolve again, so a low-TTL record could answer public for the check and
  private for the connection. Pin the connection to the validated address,
  preserving Host and TLS serverName. Opt out with pinDns: false.
- 0:0:0:0:0:ffff:127.0.0.1, ::ffff:7f00:1 and fec0::1 were not treated as
  private. Add uncompressed IPv4-mapped forms, site-local IPv6, 198.18/15
  and 192.0.0/24.

security/url.ts
- sanitizeUrl returned "//evil.com" verbatim via the relative-path fast path,
  bypassing the host checks it had just run; in an href that navigates
  cross-origin. Resolve protocol-relative input instead.

dev-server/gateway.ts
- Malformed base64 in an Authorization header threw out of checkAuth on an
  unauthenticated path. Fail closed.
- split(":", 2) truncated passwords at the first colon, so a password
  containing ":" could never authenticate.
- The credential compare short-circuited on length mismatch, leaking length
  by timing. Extracted as verifyBasicAuth so it is testable.

authz/index.ts
- Namespace wildcards only matched the first segment, so "post:comment:*"
  did not grant "post:comment:delete". Match at every depth.

uploader/operations.ts
- Validate transcoder dimensions and bitrate rather than trusting the declared
  type, and reject ".." path segments.

package.json
- The brace-expansion override pinned 5.0.8, which is inside the advisory
  range >=4.0.0 <5.0.9. Bump to 5.0.9; bun audit is now clean.

Verified: check:production passes (typecheck, lint, 1033 tests, format,
ASVS, public-API baseline, editor checks).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/authz/src/index.ts

@@ -52,11 +52,12 @@ export function defineRbac(roles: Record<string, string[]>): Rbac {
       if (!subject?.roles?.length) return false;
       const perms = permissionsFor(subject.roles);
       if (perms.has("*") || perms.has(permission)) return true;
-      // Namespace wildcards: "post:*" grants "post:write".
-      const ns = permission.includes(":")
-        ? permission.slice(0, permission.indexOf(":")) + ":*"
-        : null;
-      return ns ? perms.has(ns) : false;
+      // Namespace wildcards at every depth: "post:*" and "post:comment:*" both
+      // grant "post:comment:delete".
+      for (let at = permission.indexOf(":"); at !== -1; at = permission.indexOf(":", at + 1)) {
+        if (perms.has(`${permission.slice(0, at)}:*`)) return true;
+      }
+      return false;
     },
   };
 }