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/before_after_1230829.json

@@ -2,32 +2,32 @@
   "action_id": "before_after_1230829",
   "action_name": "Before After 1230829",
   "action": {
-    "full_body": "split view composition, side-by-side comparison, two panels showing the same character",
-    "head": "varying expressions, sad vs happy, neutral vs excited",
-    "eyes": "looking at viewer, varying intensity",
-    "arms": "neutral pose vs confident pose",
-    "hands": "hanging loosely vs gesturing",
-    "torso": "visible change in clothing or physique",
-    "pelvis": "standing or sitting",
-    "legs": "standing or sitting",
-    "feet": "neutral stance",
-    "additional": "transformation, makeover, dividing line, progression, evolution"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "F"
+    "full_body": "2koma, before_and_after",
+    "head": "heavy_breathing, orgasm, sticky_face",
+    "eyes": "eyes_closed",
+    "arms": "variation",
+    "hands": "variation",
+    "torso": "upper_body",
+    "pelvis": "variation",
+    "legs": "variation",
+    "feet": "variation",
+    "additional": "facial, bukkake, cum, cum_on_face"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/before_after_1230829.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "before_after_1230829"
+    "lora_weight": 0.9,
+    "lora_triggers": "before_after"
   },
   "tags": [
-    "split view",
-    "comparison",
-    "transformation",
-    "before and after",
-    "multiple views",
-    "concept art"
+    "before_and_after",
+    "2koma",
+    "facial",
+    "bukkake",
+    "cum",
+    "cum_on_face",
+    "orgasm",
+    "heavy_breathing",
+    "upper_body",
+    "split_theme"
   ]
 }