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/multiple_asses_r1.json

@@ -2,35 +2,29 @@
   "action_id": "multiple_asses_r1",
   "action_name": "Multiple Asses R1",
   "action": {
-    "full_body": "multiple subjects standing in a row or group turned away from the camera, full rear view",
-    "head": "facing forward away from camera or turned slightly to look back",
-    "eyes": "looking at viewer over shoulder or not visible",
-    "arms": "relaxed at sides or hands resting on hips",
-    "hands": "resting on waist or touching thighs",
-    "torso": "visible back, slightly arched lower back",
-    "pelvis": "hips wide, buttocks emphasized and focused in composition",
-    "legs": "standing straight or slightly bent at knees, shoulder-width apart",
-    "feet": "planted on ground, heels slightly raised or flat",
-    "additional": "focus on varying shapes and sizes of buttocks, depth of field focused on rear"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "multiple_girls, lineup, from_behind, ass_focus",
+    "head": "looking_back",
+    "eyes": "detailed_eyes",
+    "arms": "arms_at_sides",
+    "hands": "hands_on_hips",
+    "torso": "bent_over, arched_back",
+    "pelvis": "ass, hips",
+    "legs": "standing, legs_apart",
+    "feet": "feet_out_of_frame",
+    "additional": "group_picture, depth_of_field"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/Multiple_Asses_r1.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "Multiple_Asses_r1"
+    "lora_triggers": "Multiple_Asses_V1"
   },
   "tags": [
-    "from behind",
-    "ass",
-    "butt",
-    "multiple girls",
-    "group",
-    "back view",
-    "looking back",
-    "medium shot",
-    "lower body focus"
+    "multiple_girls",
+    "lineup",
+    "from_behind",
+    "ass_focus",
+    "3girls",
+    "bent_over",
+    "looking_back"
   ]
 }