fix(health): resolve app version in Nix bundle at runtime

The health endpoint was returning version 0.0.0 because readPackageVersion()
tried import.meta.require on a JSON path that Bun cannot bundle and the
package.json never shipped into the Nix store (installPhase only copied
dist/, home.html, schema.sql).

- flake.nix installPhase now copies package.json into $out/share/teleuploader/
- readPackageVersion() reads import.meta.dir/../package.json (bundle) or
  process.cwd()/package.json (dev) via readFileSync at runtime

Verified local resolution to 1.2.1; after this fixes deploy it will expose
the live semver on GET /health.

flake.nix

@@ -61,6 +61,9 @@
             cp -r dist $out/share/teleuploader/dist
             cp src/home.html $out/share/teleuploader/ 2>/dev/null || true
             cp schema.sql $out/share/teleuploader/ 2>/dev/null || true
+            # Carry package.json into the store so runtime version lookup in
+            # env.ts readPackageVersion() resolves (dist/../package.json).
+            cp package.json $out/share/teleuploader/package.json
 
             # Wrap with bun from Nix store (dependency sharing!)
             # Note: NO --chdir — systemd WorkingDirectory controls this