Improve documentation with comprehensive code comments and README

[staging-devin-ai-integration]

This PR adds documentation improvements as requested.

Changes in this pull request:

• Add PHPDoc class-level and method-level comments to key Model files (Account, Transaction, Budget, Bill, Category, PiggyBank, TransactionJournal, Rule) explaining their purpose, properties, and relationships in the double-entry bookkeeping system
• Add documentation to User.php explaining authentication, authorization, and user ownership of financial data
• Add documentation to base Controller and HomeController explaining their responsibilities
• Expand README with new sections: Technical Architecture, Development Setup, Project Structure, API Documentation, Database Schema, Testing, and Configuration

Items for reviewer to verify

• Accuracy of @Property annotations in PHPDoc blocks against actual database columns
• Technical details in README (PHP 8.4+, Laravel 12, API version 2.1.0) match the actual codebase requirements
• Double-entry bookkeeping explanations are technically accurate for this application's implementation

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Requested by: @samfert-codeium

Link to Devin run

[staging-devin-ai-integration]

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