refactor(config): add typed configuration schema and remove dead environment variables #689

Parent epic: #679

Evidence

Configuration is spread across env examples, validation scripts, runtime helpers, and docs. Audit examples include AUTH_JWT_SECRET despite opaque sessions, locale defaults disagreeing (id vs runtime en), variables with unclear runtime ownership, and FORM_DRAFT_RETENTION_DAYS documented without a complete example.

Objective

Create one typed configuration registry that generates validation, safe diagnostics, env examples, and reference docs.

Scope

• Inventory every env read and classify required/optional/conditional/deprecated/dead.
• Define type, default, deployment profiles, sensitivity, owner module, and validation rule once.
• Generate or verify .env.example and env reference docs from the registry.
• Deprecate misleading variables with compatibility warnings before removal.
• Keep full-online provider variables inactive unless their feature gates are enabled.

Acceptance criteria

• Every runtime env read is registered or explicitly exempted.
• Secret values never appear in validation output.
• Offline/LAN minimal config passes without external providers.
• Locale/timezone/storage sources of truth are tested.
• Deprecated variables have migration guidance and removal version.
• Config, docs, and example drift fails CI.