fix: resolve critical security vulnerabilities and reliability issues

[ClassTechStar]

Summary

This PR addresses 4 critical and 5 high-priority security vulnerabilities and reliability issues identified during a comprehensive code audit of the GalReview repository.

Critical Fixes

1. Hardcoded S3 Credentials Removed (scripts/download-practice-resources.ps1)

• Issue: AWS S3 access key (ZMVBNId4c52092lL53Jg) and secret key (upDcWNLGKlhe2qVSKOYG) were hardcoded in the repository
• Fix: Credentials now read from OSCA_ACCESS_KEY_ID and OSCA_SECRET_ACCESS_KEY environment variables

2. Timing Attack in FileService (backend/FileService/Program.cs, InternalServiceAccessPolicy.cs)

• Issue: GatewayUser() and HasSingleExactValue() used == / string.Equals() for gateway key comparison, vulnerable to timing side-channel attacks
• Fix: Replaced with CryptographicOperations.FixedTimeEquals() for constant-time comparison

3. Plaintext Admin Password (backend/AuthService/Program.cs, appsettings.Development.json)

• Issue: Admin password stored as plaintext in configuration ("Password": "admin")
• Fix: Now supports Admin:PasswordHash using ASP.NET Core Identity PasswordHasher (PBKDF2-HMAC-SHA256). Legacy Admin:Password is supported as fallback for migration. compose.integration.yaml updated to use Admin__PasswordHash.

4. Unauthenticated OCRService (backend/OCRService/app.py)

• Issue: OCRService had no authentication mechanism — any network client could call its endpoints
• Fix: Added X-Gateway-Key authentication middleware using hmac.compare_digest() for constant-time validation. FileService updated to send the key header. compose.integration.yaml updated to pass GATEWAY_KEY to ocr-service.

High-Priority Fixes

5. Weak Password Reset Token (backend/AuthService/Program.cs)

• Issue: 6-digit numeric reset token (~20 bits entropy, 1M combinations)
• Fix: 8-character alphanumeric token from unambiguous alphabet (no 0/O/1/I), ~39 bits entropy. Case-insensitive normalization for user convenience.

6. Email Enumeration (backend/AuthService/Program.cs)

• Issue: Password reset endpoint returned 404 for unregistered emails, allowing account enumeration
• Fix: Always returns 202 Accepted regardless of whether the email exists

7. PDF DoS Vulnerability (backend/OCRService/app.py)

• Issue: No page count limit on PDF processing — a malicious PDF could exhaust server resources
• Fix: Added MAX_PDF_PAGES = 50 limit with HTTP 413 response

8. Exception Message Leakage (backend/GalGameService/Program.cs)

• Issue: Raw exception messages were exposed to clients via ApiError on job failure
• Fix: Replaced with generic user-facing message; exception details still logged server-side

9. Non-Atomic Cross-Collection Write (backend/GalGameService/MongoGameStore.cs)

• Issue: SavePackage() wrote to 3 MongoDB collections (packages, manifests, owners) without a transaction — partial failures could leave inconsistent state
• Fix: Wrapped writes in a MongoDB transaction with graceful fallback to sequential writes for standalone MongoDB instances without replica set support

Files Changed

File	Changes
scripts/download-practice-resources.ps1	Remove hardcoded S3 credentials
backend/FileService/Program.cs	Fix timing attack in GatewayUser, pass gateway key to OCR
backend/FileService/InternalServiceAccessPolicy.cs	Fix timing attack in key comparison
backend/AuthService/Program.cs	Hashed admin password, stronger reset token, fix email enumeration
backend/AuthService/appsettings.Development.json	Use password hash instead of plaintext
backend/AuthService/InMemoryRepositories.cs	Update mock to match new token format
backend/OCRService/app.py	Add gateway key auth, PDF page limit
backend/GalGameService/Program.cs	Fix exception message leakage
backend/GalGameService/MongoGameStore.cs	Atomic SavePackage via transactions
compose.integration.yaml	Pass GATEWAY_KEY to ocr-service, use Admin__PasswordHash

Testing

• All changes are backward-compatible with appropriate fallback mechanisms
• GPG-signed commit for verified provenance
• No breaking API changes — all existing endpoints maintain their contracts
• Mock mode test fixtures updated to match new token format