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>

api/.dockerignore

@@ -12,6 +12,3 @@ dist/
 .env
 .env.*
 !.env.example
-tests/
-alembic/
-alembic.ini