Fix OpenAPI spec inconsistencies

- Read version dynamically from package.json instead of hardcoding 0.3.0
- Remove dead 'Signup' tag (free signup was removed)
- Add missing 'cache' parameter to POST /v1/screenshot body schema
- Add comprehensive tests to prevent regression

The cache bypass logic was already working correctly with POST body parameters.

Tests: 6/6 OpenAPI tests passing, 461/470 total tests passing (9 failing tests unrelated - missing blog post file)

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "snapapi",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "snapapi",
-      "version": "0.8.0",
+      "version": "0.9.0",
       "dependencies": {
         "compression": "^1.8.1",
         "express": "^4.21.0",