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/mushroom.json

@@ -1,34 +1,33 @@
 {
   "scene_id": "mushroom",
   "scene_name": "Mushroom",
-  "description": "A whimsical fantasy forest floor teeming with giant bioluminescent mushrooms and magical spores floating in the air.",
+  "description": "A fantasy forest landscape filled with large, glowing mushrooms and bioluminescent flora under a night sky",
   "scene": {
-    "background": "forest, trees, dense vegetation, moss, ferns, dark background, bokeh, nature, outdoors, mist",
-    "foreground": "mushroom, giant mushroom, glowing fungus, spores, sparkling, intricate details, dew drops, mycelium",
-    "furniture": [
-      "tree stump",
-      "fallen log"
-    ],
+    "background": "forest, giant_mushroom, glowing_mushroom, night, bioluminescence, nature, trees",
+    "foreground": "grass, mushroom, fungus, ferns, wildflowers",
+    "furniture": [],
     "colors": [
-      "forest green",
-      "bioluminescent blue",
-      "earthy brown",
-      "glowing purple"
+      "colorful",
+      "blue",
+      "green",
+      "purple"
     ],
-    "lighting": "bioluminescence, soft atmospheric lighting, volumetric lighting, god rays, low light, cinematic",
-    "theme": "fantasy, magical, botanical, surreal, cottagecore"
+    "lighting": "bioluminescence, glowing, cinematic_lighting",
+    "theme": "fantasy, magical, surreal"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Mushroom.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "Mushroom"
+    "lora_weight": 0.7,
+    "lora_triggers": "mshr00m"
   },
   "tags": [
+    "forest",
+    "mushroom",
     "nature",
+    "giant_mushroom",
+    "glowing_mushroom",
+    "bioluminescence",
     "fantasy",
-    "scenery",
-    "no humans",
-    "masterpiece",
-    "best quality"
+    "night"
   ]
 }