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/actions/cat_stretchill.json

@@ -2,34 +2,33 @@
   "action_id": "cat_stretchill",
   "action_name": "Cat Stretchill",
   "action": {
-    "full_body": "character on all fours performing a deep cat stretch, kneeling with chest pressed to the floor and hips raised high",
-    "head": "chin resting on the floor, looking forward or to the side",
-    "eyes": "looking up or generally forward",
-    "arms": "fully extended forward along the ground",
-    "hands": "palms flat on the floor",
-    "torso": "deeply arched back, chest touching the ground",
-    "pelvis": "elevated high in the air, creating a steep angle with the spine",
-    "legs": "kneeling, bent at the knees",
-    "feet": "toes touching the ground",
-    "additional": "emphasizes the curve of the spine and flexibility"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "F"
+    "full_body": "kneeling, all_fours, cat_stretch, pose",
+    "head": "looking_ahead, head_down",
+    "eyes": "closed_eyes, trembling",
+    "arms": "outstretched_arms, reaching_forward, hands_on_ground",
+    "hands": "palms_down",
+    "torso": "arched_back, chest_down",
+    "pelvis": "hips_up, buttocks_up",
+    "legs": "kneeling, knees_on_ground",
+    "feet": "feet_up",
+    "additional": "cat_ears, cat_tail, trembling"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/cat_stretchILL.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "cat_stretchILL"
+    "lora_weight": 0.7,
+    "lora_triggers": "stretching, cat stretch, downward dog, trembling"
   },
   "tags": [
-    "all fours",
-    "ass up",
+    "cat_stretch",
     "kneeling",
-    "cat pose",
+    "all_fours",
     "stretching",
-    "arms extended",
-    "on floor",
-    "arched back"
+    "arched_back",
+    "outstretched_arms",
+    "hands_on_ground",
+    "downward_dog",
+    "trembling",
+    "cat_ears",
+    "cat_tail"
   ]
 }