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/pov_cowgirl_looking_down_illustrious_000005.json

@@ -2,34 +2,31 @@
   "action_id": "pov_cowgirl_looking_down_illustrious_000005",
   "action_name": "Pov Cowgirl Looking Down Illustrious 000005",
   "action": {
-    "full_body": "cowgirl position, straddling, sitting on viewer, female on top",
-    "head": "looking down, face mostly visible",
-    "eyes": "looking at viewer, eye contact",
-    "arms": "arms resting on thighs or reaching down",
-    "hands": "hands on knees or hands on viewer",
-    "torso": "leaning forward, viewed from below",
-    "pelvis": "hips wide, straddling camera, intimate proximity",
-    "legs": "kneeling, knees apart, bent knees, thighs framing view",
-    "feet": "tucked behind or out of frame",
-    "additional": "pov, from below, foreshortening, depth of field"
-  },
-  "participants": {
-    "solo_focus": "true",
-    "orientation": "MF"
+    "full_body": "cowgirl_position, straddling, girl_on_top",
+    "head": "looking_down",
+    "eyes": "looking_at_viewer",
+    "arms": "reaching_towards_viewer",
+    "hands": "open_hands",
+    "torso": "leaning_forward, breasts_hanging",
+    "pelvis": "straddling",
+    "legs": "kneeling, spread_legs",
+    "feet": "toes_curled",
+    "additional": "pov, from_below, intense_glare, blushing"
   },
   "lora": {
     "lora_name": "Illustrious/Poses/Pov_Cowgirl_Looking_Down_Illustrious-000005.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "Pov_Cowgirl_Looking_Down_Illustrious-000005"
+    "lora_triggers": "CG_LD, pov, girl_on_top, straddling, leaning_forward, looking_down"
   },
   "tags": [
-    "pov",
-    "cowgirl position",
+    "cowgirl_position",
     "straddling",
-    "looking down",
-    "from below",
-    "female on top",
+    "girl_on_top",
+    "pov",
+    "looking_down",
+    "from_below",
+    "leaning_forward",
     "kneeling",
-    "intimate"
+    "reaching_towards_viewer"
   ]
 }