feat(images): store images' own categories in the library database (#332)

One new table, image_categories, and its category index, created empty
by a forward-only migration that reads and writes only its own objects.
Startup checks the structure; restore also checks the values. The store
sets, clears and reads own categories by file key (the deepest output
source holding the file and the path below it), and copies a removed
duplicate's category to a kept copy that has none.

tests/browser/workspace_visual_fixture.py

@@ -119,6 +119,7 @@ def seed_workspace_visual(api: PromptManagerAPI, root: Path) -> None:
             "media_image_tags",
             "media_annotation_bytes",
             "media_annotation_files",
+            "image_categories",
             "image_prompt_pairs",
             "prompt_pair_tags",
             "prompt_pairs",