Refactor PSD extraction: simplify to latest-only + add R2 support

## Key Changes

1. **Simplified extraction logic**
   - Changed from downloading 220+ historical archives to checking only latest available month
   - Tries current month and falls back up to 3 months (handles USDA publication lag)
   - Architecture advisor insight: ETags naturally deduplicate, historical year/month structure was unnecessary

2. **Flat storage structure**
   - Old: `data/{year}/{month}/{etag}.zip`
   - New: `data/{etag}.zip` (local) or `psd/{etag}.zip` (R2)
   - Migrated 226 existing files to flat structure

3. **Dual storage modes**
   - **Local mode**: Downloads to local directory (development)
   - **R2 mode**: Uploads to Cloudflare R2 (production)
   - Mode determined by presence of R2 environment variables
   - Added boto3 dependency for S3-compatible R2 API

4. **Updated raw SQLMesh model**
   - Changed pattern from `**/*.zip` to `*.zip` to match flat structure

## Benefits

- Simpler: Single file check instead of 220+ URL attempts
- Efficient: ETag-based deduplication works naturally
- Flexible: Supports both local dev and production R2 storage
- Maintainable: Removed unnecessary complexity

## Testing

- ✅ Local extraction works and respects ETags
- ✅ Falls back correctly when current month unavailable
- ✅ Linting passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

transform/sqlmesh_materia/models/raw/psd_data.sql

@@ -21,4 +21,4 @@ MODEL (
 )
 );
 SELECT *
- FROM read_csv('zip://extract/psdonline/src/psdonline/data/**/*.zip/*.csv', header=true, union_by_name=true, filename=true, names = ['commodity_code', 'commodity_description', 'country_code', 'country_name', 'market_year', 'calendar_year', 'month', 'attribute_id', 'attribute_description', 'unit_id', 'unit_description', 'value'], all_varchar=true)
+ FROM read_csv('zip://extract/psdonline/src/psdonline/data/*.zip/*.csv', header=true, union_by_name=true, filename=true, names = ['commodity_code', 'commodity_description', 'country_code', 'country_name', 'market_year', 'calendar_year', 'month', 'attribute_id', 'attribute_description', 'unit_id', 'unit_description', 'value'], all_varchar=true)