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>

scripts/ci/generate-log-config.sh

@@ -1,11 +1,12 @@
 #!/bin/bash
-# generate-log-config.sh - Generate .env.logging from LOG_LEVEL
-# Maps a single LOG_LEVEL environment variable to per-container settings
+# generate-log-config.sh - Generate log-level environment variables
+# Maps a single LOG_LEVEL to per-container settings and writes to stdout
 #
 # Usage: ./generate-log-config.sh [LOG_LEVEL]
 #   LOG_LEVEL: DEBUG, INFO, WARN, or ERROR (default: INFO)
 #
-# Output: Creates .env.logging file with container-specific log settings
+# Output: Log configuration variables on stdout (append to .env)
+#   Example: ./generate-log-config.sh INFO >> .env
 #
 # Exit codes:
 #   0 - Configuration generated successfully
@@ -43,27 +44,13 @@ case "$LOG_LEVEL" in
   ERROR) REDIS_LOGLEVEL="warning" ;;
 esac
 
-# Generate .env.logging file
-cat > .env.logging << EOF
-# Generated by generate-log-config.sh - DO NOT EDIT MANUALLY
-# Regenerate with: ./scripts/ci/generate-log-config.sh $LOG_LEVEL
-LOG_LEVEL=$LOG_LEVEL
+# Output log configuration to stdout
+cat << EOF
 
-# Backend/OCR (Pino)
+# Log levels (generated by generate-log-config.sh $LOG_LEVEL)
 BACKEND_LOG_LEVEL=$LOG_LEVEL_LOWER
-
-# Frontend (Vite)
-VITE_LOG_LEVEL=$LOG_LEVEL_LOWER
-
-# PostgreSQL
+TRAEFIK_LOG_LEVEL=$LOG_LEVEL
 POSTGRES_LOG_STATEMENT=$POSTGRES_LOG_STATEMENT
 POSTGRES_LOG_MIN_DURATION=$POSTGRES_LOG_MIN_DURATION
-
-# Redis
 REDIS_LOGLEVEL=$REDIS_LOGLEVEL
-
-# Traefik
-TRAEFIK_LOG_LEVEL=$LOG_LEVEL
 EOF
-
-echo "Generated .env.logging with LOG_LEVEL=$LOG_LEVEL"