v1.2.0: Prefer Gemini 2.x models, improve cover generation and Docker health

Model selection (ai.py):
- get_optimal_model() now scores Gemini 2.5 > 2.0 > 1.5 when ranking candidates
- get_default_models() fallbacks updated to gemini-2.0-pro-exp (logic) and gemini-2.0-flash (writer/artist)
- AI selection prompt rewritten: includes Gemini 2.x pricing context, guidance to avoid 'thinking' models for writer/artist roles, and instructions to prefer 2.x over 1.5
- Added image_model_name and image_model_source globals for UI visibility
- init_models() now reads MODEL_IMAGE_HINT; tries imagen-3.0-generate-001 then imagen-3.0-fast-generate-001 on both Gemini API and Vertex AI paths

Cover generation (marketing.py):
- Fixed display bug: "Attempt X/5" now correctly reads "Attempt X/3"
- Added imagen-3.0-fast-generate-001 as intermediate fallback before legacy Imagen 2
- Quality threshold: images with score < 5 are only kept if nothing better exists
- Smarter prompt refinement on retry: deformity, blur, and watermark critique keywords each append targeted corrections to the art prompt
- Fixed missing sys import (sys.platform check for macOS was silently broken)

Config / Docker:
- config.py: added MODEL_IMAGE_HINT env var, bumped version to 1.2.0
- docker-compose.yml: added MODEL_IMAGE environment variable
- Dockerfile: added libpng-dev and libfreetype6-dev for better font/PNG rendering; added HEALTHCHECK so Portainer detects unhealthy containers

System status UI:
- system_status.html: added Image row showing active Imagen model and provider (Gemini API / Vertex AI)
- Added cache expiry countdown with colour-coded badges

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.py

@@ -11,9 +11,10 @@ def get_clean_env(key, default=None):
 API_KEY = get_clean_env("GEMINI_API_KEY")
 GCP_PROJECT = get_clean_env("GCP_PROJECT")
 GCP_LOCATION = get_clean_env("GCP_LOCATION", "us-central1")
-MODEL_LOGIC_HINT = get_clean_env("MODEL_LOGIC", "AUTO") 
+MODEL_LOGIC_HINT = get_clean_env("MODEL_LOGIC", "AUTO")
 MODEL_WRITER_HINT = get_clean_env("MODEL_WRITER", "AUTO")
 MODEL_ARTIST_HINT = get_clean_env("MODEL_ARTIST", "AUTO")
+MODEL_IMAGE_HINT = get_clean_env("MODEL_IMAGE", "AUTO")
 DEFAULT_BLUEPRINT = "book_def.json"
 
 # --- SECURITY & ADMIN ---
@@ -64,4 +65,4 @@ LENGTH_DEFINITIONS = {
 }
 
 # --- SYSTEM ---
-VERSION = "1.1.0"
+VERSION = "1.2.0"