chore: add new validation rules

[tipogi]

Summary

Adds comprehensive validation rules across all data models, refactors constants organization, and separates sanitization from validation logic.

Key Changes

Code Organization

• Created src/constants.rs to centralize all constants
• Moved tag validation constants to shared location

Validation Enhancements

Tags:

• Reusable sanitize_tag_label() and validate_tag_label() functions
• Shared validation between PubkyAppTag and PubkyAppFeed

Feeds:

• Maximum 5 tags per feed
• Tag validation using shared functions

Posts:

• Attachment validation: max 10 attachments, max 200 chars per URL
• Protocol whitelist: pubky://, http://, https://, ipfs://, ipns://
• Reserved keyword protection: prevents [DELETED] content

Users:

• Image URL format validation
• Enhanced link validation (empty checks, format validation)

Sanitization vs Validation

• Before: Sanitization truncated fields exceeding limits
• After: Sanitization only normalizes (trim, lowercase), validation enforces limits

Problems Solved

1. Missing validation rules (attachments, tag counts, protocols)
2. Code duplication in tag validation
3. Mixed sanitization/validation logic
4. Poor error messages
5. Insufficient test coverage

Files Changed

• src/constants.rs (new) - Centralized constants
• src/models/tag.rs - Reusable validation functions
• src/models/feed.rs - Tag validation
• src/models/post.rs - Attachment & content validation
• src/models/user.rs - Enhanced field validation
• src/models/blob.rs, file.rs - Test improvements
• src/common.rs, lib.rs, utils.rs, uri_parser.rs - Import updates

Impact

✅ Better data integrity | ✅ Clearer error messages | ✅ Reusable validation functions | ✅ Comprehensive tests

⚠️ Breaking: Sanitization no longer truncates - validation must be called separately

[tipogi]

Thanks @catch-21 for the help!

[ok300]

I think this fixes #35