feat: implement image tools (compress, resize, convert) and PDF tools (merge, split, images-to-pdf)

Image tools now actually process images:
- Compress: re-encode JPEG with configurable quality
- Resize: Lanczos3 resize with fit modes (inside/fill/crop)
- Convert: cross-format conversion (JPEG/PNG/WebP/GIF/BMP)

PDF tools:
- Merge: combine multiple PDFs into one document
- Split: extract page ranges via pagespec (1-3,5,7-9)
- Images to PDF: embed images as JPEG in A4 pages with scaling
- Compress: re-save PDF with optimized cross-reference table

Also fix metrics format to emit default zero values.

Co-Authored-By: Kilo <kilo@kilo.ai>

backend/workers/src/pdf/compress.rs

@@ -0,0 +1,20 @@
+use std::path::{Path, PathBuf};
+
+use lopdf::Document;
+
+use crate::nats::progress::ProgressReporter;
+
+/// Compress PDF by re-saving with compression.
+pub async fn process(
+    input_path: &Path,
+    _options: &serde_json::Value,
+    output_dir: &PathBuf,
+    _progress: &ProgressReporter,
+) -> Result<PathBuf, Box<dyn std::error::Error + Send + Sync>> {
+    let mut doc = Document::load(input_path)?;
+
+    let output_path = output_dir.join(format!("compressed_{}", input_path.file_name().unwrap_or_default().to_string_lossy()));
+    doc.save_to(&mut std::fs::File::create(&output_path)?)?;
+
+    Ok(output_path)
+}