Add feature-profile endpoint and env-driven optional router profiles

[AJaySi]

Motivation

• Provide runtime visibility and a simple opt-in mechanism to control which optional routers are mounted at startup, while preserving existing behavior when the env var is not provided.
• Allow a lean deployment profile to reduce startup surface area for constrained environments without changing existing route contracts or API paths.

Description

• Added ALWRITY_FEATURE_PROFILE handling to RouterManager with a default of all and an _optional_module_matrix that defines all and default profiles.
• Updated include_optional_routers to conditionally mount optional routers based on the active profile and added a get_feature_profile_status() helper that reports the active profile and enabled modules.
• Exposed GET /api/feature-profile/status in both backend entrypoints (backend/main.py and backend/app.py) to return the active profile and enabled optional modules.
• Documented the new ALWRITY_FEATURE_PROFILE environment variable in backend/README.md and preserved existing route contracts and paths.

Testing

• Ran python -m py_compile backend/alwrity_utils/router_manager.py backend/main.py backend/app.py and compilation succeeded.
• No new automated tests were added; existing behavior is preserved for the default profile all.

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Codex Task