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/sandwich_v3_frontback_il_1164907.json

@@ -0,0 +1,32 @@
+{
+  "action_id": "sandwich_v3_frontback_il_1164907",
+  "action_name": "Sandwich V3 Frontback Il 1164907",
+  "action": {
+    "full_body": "sandwiched between two people, standing or lying, one partner in front and one behind",
+    "head": "forced backward or looking up, blushing, scared or grinning expression",
+    "eyes": "open",
+    "arms": "restrained, held back, or being grabbed by partners",
+    "hands": "restricted movement",
+    "torso": "compressed between two bodies",
+    "pelvis": "engaged in sexual activity from front and back simultaneously",
+    "legs": "standing or spread if lying",
+    "feet": "planted or passive",
+    "additional": "pincer maneuver, asymmetrical positioning"
+  },
+  "lora": {
+    "lora_name": "Illustrious/Poses/Sandwich_v3_frontback IL_1164907.safetensors",
+    "lora_weight": 0.8,
+    "lora_triggers": "fdom_sw"
+  },
+  "tags": [
+    "sandwiched",
+    "threesome",
+    "group_sex",
+    "ffm_threesome",
+    "femdom",
+    "1boy",
+    "2girls",
+    "restrained",
+    "grabbing_another's_arm"
+  ]
+}