fix(modpack-checker): Code review fixes — license, safety, and polish

Fixes 10 issues from full code review:
- License corrected from MIT to Commercial
- Deprecated datetime.utcnow() replaced with timezone-aware alternative
- PHP array bounds checks added for all platform API responses
- Modrinth file detection now derives project slug instead of using MC version
- validate_api_key() no longer swallows network errors
- HTTP timeouts added to all external API calls in PHP
- Empty API key rejection added to CLI
- Corrupted config now warns on stderr instead of failing silently
- Error response format made consistent across controller
- Docs updated with correct repo URL and clearer CurseForge ID instructions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/modpack-version-checker/src/modpack_checker/cli.py

@@ -80,8 +80,12 @@ def config_group() -> None:
 @click.argument("api_key")
 def config_set_key(api_key: str) -> None:
     """Save your CurseForge API key and validate it."""
+    api_key = api_key.strip()
+    if not api_key:
+        console.print("[red]Error:[/red] API key cannot be empty.")
+        sys.exit(1)
     cfg = Config.load()
-    cfg.curseforge_api_key = api_key.strip()
+    cfg.curseforge_api_key = api_key
     cfg.save()
 
     client = CurseForgeClient(api_key)