fix: dedup race, cwd-relative image paths, unencoded legacy URLs

- SQLite dedup race: add UNIQUE(collection_name, file_hash) and switch
  save_document to a plain INSERT. add_document now catches the
  IntegrityError from a concurrent add of the same content, cleans up its
  managed files, and returns the winning doc_id — instead of two doc_ids
  for one file (each having paid for its own LLM indexing).

- PDF image paths: store the absolute path to each extracted image
  instead of a path relative to the indexing process's cwd. The
  ![image](...) references broke as soon as a query ran from a different
  directory.

- LegacyCloudAPI: URL-encode doc_id / retrieval_id path segments (added
  _enc()), matching CloudBackend. An id containing '/', '?', '#' or a
  space previously hit the wrong endpoint or produced a malformed URL.

Adds regression tests: UNIQUE enforcement, the add-race resolving to the
winner, absolute image paths, and encoded legacy URLs.

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

tests/test_pdf_parser.py

@@ -27,3 +27,33 @@ def test_parse_nodes_are_flat_without_level():
         assert node.tokens >= 0
         assert node.index is not None
         assert node.level is None
+
+
+def test_image_paths_are_absolute(tmp_path):
+    """Image references must be absolute so they resolve regardless of cwd
+    (cwd-relative paths broke after the query ran from another directory)."""
+    import os
+    import pymupdf
+    from pageindex.parser.pdf import PdfParser
+
+    # Build a 1-page PDF with an embedded image (>= _MIN_IMAGE_SIZE).
+    pix = pymupdf.Pixmap(pymupdf.csRGB, pymupdf.IRect(0, 0, 64, 64), False)
+    pix.clear_with(128)
+    png = tmp_path / "img.png"
+    pix.save(str(png))
+
+    doc = pymupdf.open()
+    page = doc.new_page()
+    page.insert_image(pymupdf.Rect(20, 20, 180, 180), filename=str(png))
+    pdf_path = tmp_path / "withimg.pdf"
+    doc.save(str(pdf_path))
+    doc.close()
+
+    images_dir = tmp_path / "out" / "images"
+    result = PdfParser().parse(str(pdf_path), images_dir=str(images_dir))
+
+    img_paths = [im["path"] for n in result.nodes if n.images for im in n.images]
+    assert img_paths, "expected at least one extracted image"
+    for p in img_paths:
+        assert os.path.isabs(p), f"image path not absolute: {p}"
+        assert os.path.exists(p), f"image path does not resolve: {p}"