chore: centralize docker-compose variables into .env

Stripe Price IDs were hardcoded and duplicated across 4 compose files.
Log levels were hardcoded per-overlay instead of using generate-log-config.sh.
This refactors all environment-specific variables into a single .env file
that CI/CD generates from Gitea repo variables + generate-log-config.sh.

- Add .env.example template with documented variables
- Replace hardcoded values with ${VAR:-default} substitution in base compose
- Simplify prod overlay from 90 to 32 lines (remove redundant env blocks)
- Add YAML anchors to blue-green overlay (eliminate blue/green duplication)
- Remove redundant OCR env block from staging overlay
- Change generate-log-config.sh to output to stdout (pipe into .env)
- Update staging/production CI/CD to generate .env with Stripe + log vars
- Remove dangerous pk_live_ default from VITE_STRIPE_PUBLISHABLE_KEY

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

docker-compose.staging.yml

@@ -63,27 +63,6 @@ services:
   mvp-ocr:
     image: ${OCR_IMAGE:-git.motovaultpro.com/egullickson/ocr:latest}
     container_name: mvp-ocr-staging
-    environment:
-      # Python log levels: DEBUG | INFO | WARNING | ERROR | CRITICAL
-      LOG_LEVEL: debug
-      REDIS_HOST: mvp-redis
-      REDIS_PORT: 6379
-      REDIS_DB: 1
-      # OCR engine configuration (Google Vision primary, PaddleOCR fallback)
-      OCR_PRIMARY_ENGINE: google_vision
-      OCR_FALLBACK_ENGINE: paddleocr
-      OCR_CONFIDENCE_THRESHOLD: "0.6"
-      OCR_FALLBACK_THRESHOLD: "0.6"
-      GOOGLE_VISION_KEY_PATH: /run/secrets/google-wif-config.json
-      VISION_MONTHLY_LIMIT: "1000"
-      # Vertex AI / Gemini configuration (maintenance schedule extraction)
-      VERTEX_AI_PROJECT: motovaultpro
-      VERTEX_AI_LOCATION: us-central1
-      GEMINI_MODEL: gemini-2.5-flash
-    volumes:
-      - ./secrets/app/auth0-ocr-client-id.txt:/run/secrets/auth0-ocr-client-id:ro
-      - ./secrets/app/auth0-ocr-client-secret.txt:/run/secrets/auth0-ocr-client-secret:ro
-      - ./secrets/app/google-wif-config.json:/run/secrets/google-wif-config.json:ro
 
   # ========================================
   # PostgreSQL (Staging - Separate Database)