Add Checkpoints Gallery with per-checkpoint generation settings

- New Checkpoint model (slug, name, checkpoint_path, data JSON, image_path)
- sync_checkpoints() loads metadata from data/checkpoints/*.json and falls
  back to template defaults for models without a JSON file
- _apply_checkpoint_settings() applies per-checkpoint steps, CFG, sampler,
  base positive/negative prompts, and VAE (with dynamic VAELoader node
  injection for non-integrated VAEs) to the ComfyUI workflow
- Bulk Create from Checkpoints: scans Illustrious/Noob model directories,
  reads matching HTML files, uses LLM to populate metadata, falls back to
  template defaults when no HTML is present
- Gallery index with batch cover generation and WebSocket progress bar
- Detail page showing Generation Settings and Base Prompts cards
- Checkpoints nav link added to layout
- New data/prompts/checkpoint_system.txt LLM system prompt
- Updated README with all current galleries and file structure
- Also includes accumulated action/scene JSON updates, new actions, and
  other template/generator improvements from prior sessions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

data/scenes/space_backround_illustrious.json

@@ -1,30 +1,32 @@
 {
   "scene_id": "space_backround_illustrious",
   "scene_name": "Space Backround Illustrious",
-  "description": "A breathtaking view of a vibrant, swirling nebula and distant galaxies in deep space.",
+  "description": "A majestic and aesthetic space scenery featuring intricate starry skies, vibrant nebulae, and the vast universe, designed to serve as a high-quality background for illustrations.",
   "scene": {
-    "background": "outer space, nebula, galaxy, milky way, starry sky, cosmic dust, purple space, blue space, hyperdetailed sky",
-    "foreground": "no humans, scenery, wide shot, lens flare",
+    "background": "space, starry_sky, universe, nebula, galaxy, milky_way, scenery",
+    "foreground": "",
     "furniture": [],
     "colors": [
-      "deep violet",
-      "cyan",
-      "midnight blue",
-      "glowing magenta"
+      "purple_theme",
+      "blue_theme",
+      "black"
     ],
-    "lighting": "bioluminescent, starlight, cinematic lighting, chromatic aberration",
-    "theme": "sci-fi, celestial, atmospheric, mysterious"
+    "lighting": "starry_sky",
+    "theme": "science_fiction"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/space_backround_illustrious.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "space_backround_illustrious"
+    "lora_weight": 0.8,
+    "lora_triggers": "space_backround"
   },
   "tags": [
-    "masterpiece",
-    "best quality",
-    "wallpaper",
-    "astrophotography",
-    "concept art"
+    "space",
+    "universe",
+    "galaxy",
+    "nebula",
+    "starry_sky",
+    "scenery",
+    "science_fiction",
+    "milky_way"
   ]
 }