feat(autocomplete): add Danbooru/e621 tag search with category filtering

- Add TagFTSIndex service for fast SQLite FTS5-based tag search (221k+ tags)
- Implement command-mode autocomplete: /char, /artist, /general, /meta, etc.
- Support category filtering via category IDs or names
- Return enriched results with post counts and category badges
- Add UI styling for category badges and command list dropdown

docs/reference/danbooru_e621_categories.md

@@ -0,0 +1,69 @@
+# Danbooru/E621 Tag Categories Reference
+
+Reference for category values used in `danbooru_e621_merged.csv` tag files.
+
+## Category Value Mapping
+
+### Danbooru Categories
+
+| Value | Description |
+|-------|-------------|
+| 0 | General |
+| 1 | Artist |
+| 2 | *(unused)* |
+| 3 | Copyright |
+| 4 | Character |
+| 5 | Meta |
+
+### e621 Categories
+
+| Value | Description |
+|-------|-------------|
+| 6 | *(unused)* |
+| 7 | General |
+| 8 | Artist |
+| 9 | Contributor |
+| 10 | Copyright |
+| 11 | Character |
+| 12 | Species |
+| 13 | *(unused)* |
+| 14 | Meta |
+| 15 | Lore |
+
+## Danbooru Category Colors
+
+| Description | Normal Color | Hover Color |
+|-------------|--------------|-------------|
+| General | #009be6 | #4bb4ff |
+| Artist | #ff8a8b | #ffc3c3 |
+| Copyright | #c797ff | #ddc9fb |
+| Character | #35c64a | #93e49a |
+| Meta | #ead084 | #f7e7c3 |
+
+## CSV Column Structure
+
+Each row in the merged CSV file contains 4 columns:
+
+| Column | Description | Example |
+|--------|-------------|---------|
+| 1 | Tag name | `1girl`, `highres`, `solo` |
+| 2 | Category value (0-15) | `0`, `5`, `7` |
+| 3 | Post count | `6008644`, `5256195` |
+| 4 | Aliases (comma-separated, quoted) | `"1girls,sole_female"`, empty string |
+
+### Sample Data
+
+```
+1girl,0,6008644,"1girls,sole_female"
+highres,5,5256195,"high_res,high_resolution,hires"
+solo,0,5000954,"alone,female_solo,single,solo_female"
+long_hair,0,4350743,"/lh,longhair"
+mammal,12,3437444,"cetancodont,cetancodontamorph,feralmammal"
+anthro,7,3381927,"adult_anthro,anhtro,antho,anthro_horse"
+skirt,0,1557883,
+```
+
+## Source
+
+- [PR #312: Add danbooru_e621_merged.csv](https://github.com/DominikDoom/a1111-sd-webui-tagcomplete/pull/312)
+- [DraconicDragon/dbr-e621-lists-archive](https://github.com/DraconicDragon/dbr-e621-lists-archive)