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/scenicill.json

@@ -1,34 +1,42 @@
 {
   "scene_id": "scenicill",
   "scene_name": "Scenicill",
-  "description": "A breathtaking fantasy landscape illustrating a vast valley with floating islands and cascading waterfalls under a vibrant sunset.",
+  "description": "A mystical and detailed forest landscape bathed in the soft glow of twilight and starlight, evoking a magical atmosphere.",
   "scene": {
-    "background": "scenery, outdoors, sky, clouds, day, mountain, waterfall, fantasy, tree, nature, water, river, floating_island, sunset, horizon, vast, panoramic view, aesthetic",
-    "foreground": "grass, flower, field, moss, rock, path, depth of field, detailed flora",
+    "background": "scenery, outdoors, nature, forest, tree, night, starry_sky, twilight, magic",
+    "foreground": "grass, flower, fireflies, glowing, no_humans",
     "furniture": [
-      "ancient ruins",
-      "stone archway"
+      "rock",
+      "tree_stump"
     ],
     "colors": [
-      "gold",
-      "azure",
-      "cyan",
-      "verdant green"
+      "blue_theme",
+      "purple_theme"
     ],
-    "lighting": "cinematic lighting, volumetric lighting, god rays, glimmering, golden hour",
-    "theme": "fantasy, adventure, majesty, tranquility"
+    "lighting": "starry_sky, glowing, cinematic_lighting",
+    "theme": "fantasy, magic"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/ScenicILL.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "ScenicILL"
+    "lora_weight": 0.8,
+    "lora_triggers": "scenery"
   },
   "tags": [
-    "wallpaper",
-    "highly detailed",
+    "scenery",
+    "no_humans",
+    "nature",
+    "outdoors",
+    "forest",
+    "tree",
+    "grass",
+    "flower",
+    "night",
+    "twilight",
+    "starry_sky",
+    "glowing",
+    "fireflies",
+    "magic",
     "masterpiece",
-    "best quality",
-    "vivid colors",
-    "painting"
+    "best_quality"
   ]
 }