Feat: Gate API docs endpoints behind API_DOCS_ENABLED env var

When API_DOCS_ENABLED=false, FastAPI registers no routes for /docs,
/redoc, or /openapi.json, returning 404 for all three. Default is true
for backwards compatibility. Invalid values fall back to true (FR-007).

Fix: Remove tests/ and alembic/ from api/.dockerignore so the test
Dockerfile (which uses COPY . .) includes the test suite; Dockerfile.prod
is unaffected as it only copies app/ explicitly.

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

.env.example

@@ -27,3 +27,7 @@ LOGIN_COOLDOWN_SECONDS=900
 # Comma-separated IPs/CIDRs of trusted upstream proxies (e.g. nginx ingress pod CIDR).
 # Leave empty when not behind a reverse proxy.
 LOGIN_TRUSTED_PROXY_IPS=
+
+# API documentation endpoints (Swagger UI, ReDoc, OpenAPI schema)
+# Set to false in production to avoid exposing the API surface publicly.
+API_DOCS_ENABLED=true