feat(hardcover): Improve Hardcover metadata accuracy and rate limiting

[balazs-szucs]

🚀 Pull Request

📝 Description

Closes: #2244

This update improves how the application fetches book data from the Hardcover API. It focuses on better search results, accurate mood tags, and stable API usage.

🛠️ Changes Implemented

Search Improvements

• Added combined title+author search for better match accuracy
• Implemented automatic fallback to title-only search when combined search fails
• Improved author name matching using fuzzy string comparison

Mood and Tag Filtering

• Added vote-based filtering to remove low-quality mood tags
• Implemented HardcoverMoodFilter with configurable thresholds (minimum 3 votes, 15% of max)
• Fetch detailed book metadata for top result to get accurate mood counts
• Fall back to basic filtering when detailed fetch fails

API Rate Limiting

• Added adaptive rate limiting with exponential backoff
• Initial delay of 1200ms between requests, doubles on throttle errors
• Automatically reduces delay after successful requests
• Handles HTTP 429 throttle responses properly

Code Quality

• Split large method into smaller focused methods
• Added unit tests
• Better error handling for invalid dates and book IDs
• Improved null safety throughout

🧪 Testing Strategy

📸 Visual Changes (if applicable)

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⚠️ Required Pre-Submission Checklist

Please Read - This Checklist is Mandatory

Important Notice: We've experienced several production bugs recently due to incomplete pre-submission checks. To maintain code quality and prevent issues from reaching production, we're enforcing stricter adherence to this checklist.

All checkboxes below must be completed before requesting review. PRs that haven't completed these requirements will be sent back for completion.

Mandatory Requirements (please check ALL boxes):

• Code adheres to project style guidelines and conventions
• Branch synchronized with latest develop branch (please resolve any merge conflicts)
• 🚨 CRITICAL: Automated unit tests added/updated to cover changes (MANDATORY for ALL Spring Boot backend and Angular frontend changes - this is non-negotiable)
• 🚨 CRITICAL: All tests pass locally (run ./gradlew test for Spring Boot backend, and ng test for Angular frontend - NO EXCEPTIONS)
• 🚨 CRITICAL: Manual testing completed in local development environment (verify your changes work AND no existing functionality is broken - test related features thoroughly)
• Flyway migration versioning follows correct sequence (if database schema was modified)
• Documentation PR submitted to booklore-docs (required for features or enhancements that introduce user-facing or visual changes)

Why This Matters:

Recent production incidents have been traced back to:

• Incomplete testing coverage (especially backend)
• Merge conflicts not resolved before merge
• Missing documentation for new features

Backend changes without tests will not be accepted. By completing this checklist thoroughly, you're helping maintain the quality and stability of Booklore for all users.

Note to Reviewers: Please verify the checklist is complete before beginning your review. If items are unchecked, kindly ask the contributor to complete them first.

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💬 Additional Context (optional)