Create comprehensive ERD documentation and model diagrams

Problem

SwiftData model relationships and structure are not well documented:

• No Entity-Relationship Diagram (ERD)
• No visual representation of model relationships
• Difficult for new contributors to understand data model
• No documentation of delete rules and cascading behavior

Proposed Documentation

1. Create ERD Diagram

Create docs/ERD.md with Mermaid diagram showing:

• All entities (Account, Profile, Stream, Thought, BlockedProfile)
• Relationships and cardinality
• Delete rules (cascade, nullify)
• Unique constraints
• External storage attributes

2. Create Model Documentation

Create docs/MODELS.md with:

• Detailed description of each model
• Property descriptions
• Relationship explanations
• Usage examples
• Common pitfalls

3. Safe Access Patterns

Document safe vs unsafe patterns:

DO:

// Use computed properties that filter deleted objects
let validThoughts = stream.validThoughts

// Check before accessing optional relationships
if let source = stream.safeSource {
    // source is guaranteed to be valid
}

DON'T:

// Direct access without checking isDeleted
let mediaType = stream.source?.metadata.mediaType  // ⚠️ May crash!

Implementation Checklist

• Create docs/ERD.md with Mermaid diagram
• Create docs/MODELS.md with detailed docs
• Add inline documentation to model files
• Create docs/SWIFTDATA_GUIDELINES.md
• Update CONTRIBUTING.md with data model section
• Add examples of safe vs unsafe patterns
• Document migration process
• Add troubleshooting guide

Files to Create

• docs/ERD.md
• docs/MODELS.md
• docs/SWIFTDATA_GUIDELINES.md

Files to Modify

• CONTRIBUTING.md
• README.md (add link to data model docs)