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>

.env.example

@@ -0,0 +1,36 @@
+# MotoVaultPro Environment Configuration
+# Copy to .env and fill in environment-specific values
+# Generated .env files should NOT be committed to version control
+#
+# Local dev: No .env needed -- base docker-compose.yml defaults are sandbox values
+# Staging/Production: CI/CD generates .env from Gitea variables + generate-log-config.sh
+
+# ===========================================
+# Stripe Price IDs (environment-specific)
+# ===========================================
+# Sandbox defaults used for local development
+STRIPE_PRO_MONTHLY_PRICE_ID=price_1T1ZHMJXoKkh5RcKwKSSGIlR
+STRIPE_PRO_YEARLY_PRICE_ID=price_1T1ZHnJXoKkh5RcKWlG2MPpX
+STRIPE_ENTERPRISE_MONTHLY_PRICE_ID=price_1T1ZIBJXoKkh5RcKu2jyhqBN
+STRIPE_ENTERPRISE_YEARLY_PRICE_ID=price_1T1ZIQJXoKkh5RcK34YXiJQm
+
+# ===========================================
+# Stripe Publishable Key (baked into frontend at build time)
+# ===========================================
+# VITE_STRIPE_PUBLISHABLE_KEY=pk_test_...
+
+# ===========================================
+# Log Levels (generated by scripts/ci/generate-log-config.sh)
+# ===========================================
+# Run: ./scripts/ci/generate-log-config.sh DEBUG >> .env
+#
+# BACKEND_LOG_LEVEL=debug
+# TRAEFIK_LOG_LEVEL=DEBUG
+# POSTGRES_LOG_STATEMENT=all
+# POSTGRES_LOG_MIN_DURATION=0
+# REDIS_LOGLEVEL=debug
+
+# ===========================================
+# Grafana
+# ===========================================
+# GRAFANA_ADMIN_PASSWORD=admin