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/arch_back_sex_v1_1_illustriousxl.json

@@ -2,35 +2,28 @@
   "action_id": "arch_back_sex_v1_1_illustriousxl",
   "action_name": "Arch Back Sex V1 1 Illustriousxl",
   "action": {
-    "full_body": "character positioned with an exaggerated spinal curve, emphasizing the posterior",
-    "head": "thrown back in pleasure or looking back over the shoulder",
-    "eyes": "half-closed, rolled back, or heavily blushing",
-    "arms": "supporting body weight on elbows or hands, or reaching forward",
-    "hands": "gripping bedsheets or pressing firmly against the surface",
-    "torso": "extremely arched back, chest pressed down or lifted depending on angle",
-    "pelvis": "tilted upwards, hips raised high",
-    "legs": "kneeling or lying prone with knees bent",
-    "feet": "toes curled or extending upwards",
-    "additional": "sweat, blush, heavy breathing indication"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "doggystyle, sex_from_behind, all_fours",
+    "head": "head_back, looking_back",
+    "eyes": "closed_eyes, blush",
+    "arms": "arms_support",
+    "hands": "grabbing_another's_ass",
+    "torso": "arched_back",
+    "pelvis": "lifted_hip",
+    "legs": "kneeling, spread_legs",
+    "feet": "toes",
+    "additional": "kiss, sweat, saliva, intense_pleasure"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/arch-back-sex-v1.1-illustriousxl.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "arch-back-sex-v1.1-illustriousxl"
+    "lora_triggers": "kiss, arched_back, sex_from_behind"
   },
   "tags": [
-    "arch back",
-    "arched back",
-    "hips up",
-    "curved spine",
-    "ass focus",
-    "hyper-extension",
-    "prone",
-    "kneeling",
-    "from behind"
+    "hetero",
+    "sex",
+    "vaginal",
+    "penis",
+    "nude",
+    "indoors"
   ]
 }