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/regression_illustrious.json

@@ -2,31 +2,27 @@
   "action_id": "regression_illustrious",
   "action_name": "Regression Illustrious",
   "action": {
-    "full_body": "sitting on floor, curled up, huddled, hugging knees, making body appear smaller",
-    "head": "resting on knees or looking up from low angle, shy expression",
-    "eyes": "wide innocent eyes, looking up",
-    "arms": "wrapping around shins, hugging legs tight",
-    "hands": "clasped together in front of shins or hiding inside sleeves",
-    "torso": "hunched forward slightly, compacted",
-    "pelvis": "sitting flat on the ground",
-    "legs": "knees pulled up tight to chest, deep flexion",
-    "feet": "heels close to body, toes pointing inward",
-    "additional": "oversized clothing, vulnerable posture, perspective from above looking down"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "standing, full_body, small_stature",
+    "head": "looking_at_viewer, blush",
+    "eyes": "open_eyes",
+    "arms": "arms_at_sides, sleeves_past_fingers",
+    "hands": "hands_hidden",
+    "torso": "oversized_clothes",
+    "pelvis": "hips",
+    "legs": "legs_together",
+    "feet": "barefoot",
+    "additional": "age_regression, child, cute"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/Regression_Illustrious.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "Regression_Illustrious"
+    "lora_triggers": "AgeRegression"
   },
   "tags": [
-    "sitting",
-    "hugging knees",
-    "curled up",
-    "floor",
-    "vulnerable"
+    "age_regression",
+    "oversized_clothes",
+    "sleeves_past_fingers",
+    "child",
+    "cute"
   ]
 }