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>

CLAUDE.md

@@ -36,13 +36,24 @@ This is a uv workspace with three main components:
 ### 1. Extract Layer (`extract/`)
 Contains extraction packages for pulling data from external sources.
 
-- **`extract/psdonline/`**: Extracts USDA PSD commodity data from archives dating back to 2006
+- **`extract/psdonline/`**: Extracts USDA PSD commodity data
   - Entry point: `extract_psd` CLI command (defined in `extract/psdonline/src/psdonline/execute.py`)
-  - Downloads monthly zip archives to `extract/psdonline/src/psdonline/data/`
+  - Checks latest available monthly snapshot (tries current month and 3 months back)
   - Uses ETags to avoid re-downloading unchanged files
+  - Storage modes:
+    - **Local mode** (no R2 credentials): Downloads to `extract/psdonline/src/psdonline/data/{etag}.zip`
+    - **R2 mode** (R2 credentials present): Uploads to `s3://bucket/psd/{etag}.zip`
+  - Flat structure: files named by ETag for natural deduplication
 
 **Run extraction:**
 ```bash
+extract_psd  # Local mode (default)
+
+# R2 mode (requires env vars: R2_ENDPOINT, R2_BUCKET, R2_ACCESS_KEY, R2_SECRET_KEY)
+export R2_ENDPOINT=...
+export R2_BUCKET=...
+export R2_ACCESS_KEY=...
+export R2_SECRET_KEY=...
 extract_psd
 ```