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/usbreedingslave.json

@@ -2,34 +2,31 @@
   "action_id": "usbreedingslave",
   "action_name": "Usbreedingslave",
   "action": {
-    "full_body": "kneeling on all fours, submissive posture, presenting rear",
-    "head": "turned looking back over shoulder, mouth slightly open, blushing",
-    "eyes": "looking backward, upward gaze, heavy lidded",
-    "arms": "forced behind back, hyperextended shoulders",
-    "hands": "wrists bound together, hands clasped",
-    "torso": "deeply arched back, chest pressed low towards the ground",
-    "pelvis": "hips raised high, anterior pelvic tilt",
-    "legs": "kneeling, knees spread wide apart on the ground",
-    "feet": "toes curled against the surface, soles visible",
-    "additional": "wearing collar, sweat drops, heavy atmosphere"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "breeding_mount, bent_over, bound_wrists",
+    "head": "looking_at_viewer, facing_viewer",
+    "eyes": "seductive_look",
+    "arms": "bound, arms_behind_back",
+    "hands": "bound",
+    "torso": "bent_over",
+    "pelvis": "bottomless, no_pants",
+    "legs": "standing, legs_apart",
+    "feet": "standing",
+    "additional": "teasing, restrained"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/USBreedingSlave.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "USBreedingSlave"
+    "lora_weight": 0.6,
+    "lora_triggers": "USBreedingSlave, breeding_mount"
   },
   "tags": [
-    "kneeling",
-    "all fours",
-    "arched back",
-    "hands bound",
-    "from behind",
-    "submission",
-    "presenting",
-    "doggystyle"
+    "breeding_mount",
+    "bent_over",
+    "bound",
+    "restrained",
+    "bottomless",
+    "looking_at_viewer",
+    "teasing",
+    "seductive_smile",
+    "no_pants"
   ]
 }