feat: support PostgreSQL connection URL from environment

[chenyme]

What changed

• adds support for injecting the PostgreSQL connection URL through GROK2API_DATABASE_URL
• automatically selects the PostgreSQL driver when the environment variable is non-empty
• preserves existing SQLite and PostgreSQL YAML behavior when the variable is unset or empty
• accepts postgres:// and postgresql:// URLs
• rejects SQLAlchemy postgresql+asyncpg:// URLs with an actionable migration message
• redacts PostgreSQL credentials from configuration, connection, and health-check errors
• preserves the original Go error chain through Unwrap, allowing existing errors.Is and errors.As checks to continue working
• exposes the optional variable through the built-in Docker Compose configuration
• documents environment-based PostgreSQL configuration in the English, Chinese, backend, and example configuration documentation

Configuration priority

The startup configuration priority is:

built-in defaults
    < config.yaml
    < GROK2API_DATABASE_URL
    < CLI overrides

The current CLI does not provide a database override, so GROK2API_DATABASE_URL is currently the highest-priority database setting.

A non-empty environment value overrides database.postgres.dsn and automatically sets:

database:
  driver: postgres

An unset, empty, or whitespace-only value does not override YAML.

Usage

Docker Compose:

GROK2API_DATABASE_URL='postgresql://user:password@host:5432/grok2api?sslmode=require' \
docker compose up -d

Kubernetes and PaaS deployments can inject the same variable through their Secret management facilities.

Platforms that only provide DATABASE_URL can map it explicitly:

environment:
  GROK2API_DATABASE_URL: "${DATABASE_URL}"

The application intentionally does not read the generic DATABASE_URL implicitly, avoiding collisions with unrelated services or sidecars.

Compatibility

• no API changes
• no database schema or migration changes
• existing SQLite deployments are unchanged
• existing YAML-based PostgreSQL deployments are unchanged
• PostgreSQL keyword/value DSNs remain supported in YAML
• URL-only validation applies only to GROK2API_DATABASE_URL
• connection-pool settings continue to come from YAML or built-in defaults
• the environment variable is read at startup and requires a restart to change

Security

• invalid environment URLs never include the original URL or password in returned errors
• PostgreSQL driver parse, connection, and ping errors are sanitized
• URL and keyword/value password forms are redacted
• sanitized errors retain their underlying error chain for programmatic classification

Validation

• go test ./... in backend
• targeted go test -race for configuration and relational persistence
• go vet ./...
• docker compose config --quiet
• repeated environment-priority regression tests
• malformed URL, unsupported scheme, asyncpg migration, empty override, generic DATABASE_URL, and credential-redaction tests
• git diff --check

[FakerL]

Thx！