feat(storage): add S3/R2 abstraction with local Rustfs dev

- Add backend/internal/s3/ with build-tag pattern (dev vs prod)
- Dev: Uses local Rustfs container (S3-compatible)
- Prod: Stub for R2 Cloudflare (add AWS SDK to implement)
- Add S3 env vars to .env.example and .env
- Add Rustfs service to compose.yml
- Add Rustfs reset to local-dev-2.sh (wipes data on each run)

local-dev-2.sh

@@ -43,6 +43,13 @@ docker compose up postgres -d > /dev/null 2>&1
 log_success "PostgreSQL reset complete"
 sleep 3
 
+# --- 3b. Rustfs Reset (wipes data each run) ---
+log_step "Resetting Rustfs (S3-compatible storage)..."
+docker compose down -v rustfs > /dev/null 2>&1
+docker compose up rustfs -d > /dev/null 2>&1
+log_success "Rustfs reset complete"
+sleep 2
+
 # --- 4. Tmux Session Setup ---
 if tmux has-session -t $SESSION_NAME 2>/dev/null; then
   log_info "Killing existing tmux session..."