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/3p_sex_000037.json

@@ -2,32 +2,29 @@
   "action_id": "3p_sex_000037",
   "action_name": "3P Sex 000037",
   "action": {
-    "full_body": "group sex, threesome, 3 subjects, intertwined bodies, on bed",
-    "head": "pleasured expression, heavy breathing, blush, sweating, tongue out, saliva",
-    "eyes": "half-closed eyes, rolling back, heart pupils, looking at partner",
-    "arms": "wrapping around partners, arm locking, grabbing shoulders",
-    "hands": "groping, fondling, gripping hair, fingers digging into skin",
-    "torso": "arching back, bare skin, sweat drops, pressing chests together",
-    "pelvis": "connected, hips grinding, penetration",
-    "legs": "spread legs, legs in air, wrapped around waist, kneeling",
-    "feet": "toes curled, arched feet",
-    "additional": "sex act, bodily fluids, cum, motion blur, intimacy, nsfw"
-  },
-  "participants": {
-    "solo_focus": "false",
-    "orientation": "MfF"
+    "full_body": "threesome",
+    "head": "blush",
+    "eyes": "half-closed_eyes",
+    "arms": "reaching",
+    "hands": "groping",
+    "torso": "nude",
+    "pelvis": "sex",
+    "legs": "spread_legs",
+    "feet": "toes_curled",
+    "additional": "sweat"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/3P_SEX-000037.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "3P_SEX-000037"
+    "lora_weight": 0.8,
+    "lora_triggers": "threesome, group_sex"
   },
   "tags": [
-    "group sex",
     "threesome",
-    "3p",
-    "nsfw",
-    "sexual intercourse",
-    "pleasure"
+    "group_sex",
+    "nude",
+    "sex",
+    "blush",
+    "groping",
+    "sweat"
   ]
 }