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/pussy_sandwich_illustrious.json

@@ -0,0 +1,30 @@
+{
+  "action_id": "pussy_sandwich_illustrious",
+  "action_name": "Pussy Sandwich Illustrious",
+  "action": {
+    "full_body": "Two girls in a yuri embrace or interaction, one character positioning their legs to sandwich the other's face or body part",
+    "head": "Expressions of arousal or exertion, looking at partner or viewer",
+    "eyes": "Half-closed or detailed eyes",
+    "arms": "Holding legs or grabbing sheets",
+    "hands": "Touching thighs or partner's body",
+    "torso": "Twisted or leaning forward to engage in the leg lock",
+    "pelvis": "Hips positioned to squeeze or rub",
+    "legs": "Thighs squeezing tightly, engaging the pussy sandwich mechanics",
+    "feet": "Toes curled or flat depending on leverage",
+    "additional": "Intimate atmosphere, skin friction"
+  },
+  "lora": {
+    "lora_name": "Illustrious/Poses/pussy_sandwich_illustrious.safetensors",
+    "lora_weight": 1.0,
+    "lora_triggers": "pussy_sandwich"
+  },
+  "tags": [
+    "pussy_sandwich",
+    "yuri",
+    "2girls",
+    "between_legs",
+    "thighs",
+    "sandwiched",
+    "lesbian"
+  ]
+}