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/mask_pull_up.json

@@ -2,32 +2,28 @@
   "action_id": "mask_pull_up",
   "action_name": "Mask Pull Up",
   "action": {
-    "full_body": "Upper body or portrait shot of a character interacting with their face covering",
-    "head": "Facing forward, chin slightly tucked or level",
-    "eyes": "focused gaze looking directly at the viewer",
-    "arms": "Elbows bent, raised towards the face",
-    "hands": "Fingers grasping the top edge of a mask or fabric, pulling it upwards",
-    "torso": "Shoulders squared",
+    "full_body": "upper_body",
+    "head": "head_tilt, looking_at_viewer",
+    "eyes": "open_eyes",
+    "arms": "arms_up, hand_to_face",
+    "hands": "hand_on_mask",
+    "torso": "upper_body",
     "pelvis": "n/a",
     "legs": "n/a",
     "feet": "n/a",
-    "additional": "The mask fabric is taut where pulled, potentially covering the mouth and nose or in the process of doing so"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "additional": "adjusting_mask"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/mask_pull_up.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "mask_pull_up"
+    "lora_triggers": "surgical mask, mask pull"
   },
   "tags": [
-    "adjusting_mask",
+    "surgical_mask",
     "mask_pull",
     "hand_on_mask",
-    "covering_mouth",
-    "upper_body",
-    "mask"
+    "adjusting_mask",
+    "head_tilt",
+    "upper_body"
   ]
 }