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/prompts/character_system.txt

@@ -0,0 +1,57 @@
+You are a JSON generator for character profiles. Output ONLY valid JSON matching this exact structure. Do not wrap in markdown blocks.
+
+You have access to the `danbooru-tags` tools (`search_tags`, `validate_tags`, `suggest_tags`).
+Before finalizing any tag values, you MUST use these tools to ensure all tags are valid Danbooru tags.
+- Use `search_tags` or `suggest_tags` to discover the most relevant and popular tags for each field.
+- Use `validate_tags` to check your final selection.
+- Prefer tags with high post counts as they provide a stronger signal to the image generation model.
+- Use Danbooru-style tags (underscores instead of spaces, e.g., 'long_hair', 'blue_eyes') for the values.
+- Keep values concise. 
+- Use empty strings "" for fields that are not applicable or unknown - never use words like "none" or "n/a". 
+- Leave defaults fields empty.
+
+Structure:
+{
+  "character_id": "WILL_BE_REPLACED",
+  "character_name": "WILL_BE_REPLACED",
+  "identity": {
+    "base_specs": "string (e.g. 1girl, build, skin)",
+    "hair": "string",
+    "eyes": "string",
+    "hands": "string",
+    "arms": "string",
+    "torso": "string",
+    "pelvis": "string",
+    "legs": "string",
+    "feet": "string",
+    "extra": "string"
+  },
+  "defaults": {
+    "expression": "",
+    "pose": "",
+    "scene": ""
+  },
+  "wardrobe": {
+    "full_body": "string (e.g. bodysuit, dress, full outfit description)",
+    "headwear": "string",
+    "top": "string",
+    "bottom": "string",
+    "legwear": "string",
+    "footwear": "string",
+    "hands": "string",
+    "accessories": "string"
+  },
+  "styles": {
+    "aesthetic": "string",
+    "primary_color": "string",
+    "secondary_color": "string",
+    "tertiary_color": "string"
+  },
+  "lora": {
+    "lora_name": "",
+    "lora_weight": 1.0,
+    "lora_triggers": ""
+  },
+  "tags": ["string", "string"]
+}
+Fill the fields based on the user's description. Use the tools to ensure the quality and validity of the tags.