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/bentback.json

@@ -2,33 +2,28 @@
   "action_id": "bentback",
   "action_name": "Bentback",
   "action": {
-    "full_body": "standing, upper body bent forward at the waist, torso parallel to the ground",
-    "head": "turned to look back, looking over shoulder, chin raised",
-    "eyes": "looking at viewer, focused gaze",
-    "arms": "arms extended downward or resting on legs",
-    "hands": "hands on knees, hands on thighs",
-    "torso": "back arched, spinal curve emphasized, bent forward",
-    "pelvis": "hips pushed back, buttocks protruding",
-    "legs": "straight legs, locked knees or slight bend",
-    "feet": "feet shoulder-width apart, waiting",
-    "additional": "view from behind, dorsal view, dynamic posture"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "bent_over, leaning_forward, from_behind",
+    "head": "looking_at_viewer, looking_back",
+    "eyes": "open_eyes",
+    "arms": "arms_at_sides",
+    "hands": "hands_on_legs",
+    "torso": "twisted_torso, arched_back",
+    "pelvis": "ass_focus",
+    "legs": "kneepits",
+    "feet": "barefoot",
+    "additional": "unnatural_body"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/BentBack.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "BentBack"
+    "lora_triggers": "bentback, kneepits"
   },
   "tags": [
-    "bent over",
-    "standing",
-    "hands on knees",
-    "looking back",
-    "from behind",
-    "arched back",
-    "torso bend"
+    "bent_over",
+    "from_behind",
+    "looking_back",
+    "kneepits",
+    "twisted_torso",
+    "ass_focus"
   ]
 }