Refactor logging suppression with policy overrides and feature-gated video filter

[AJaySi]

Motivation

• Consolidate scattered logger suppression into a declarative, maintainable policy set to make suppression behavior easier to reason about and extend.
• Allow runtime overrides of logger levels via an environment variable so operators can relax or tighten specific loggers without changing code by using ALWRITY_LOG_LEVEL_OVERRIDES.
• Keep the Story Video focused INFO sink but make it opt-in behind a clear feature flag to avoid surprising console output by default via ALWRITY_ENABLE_STORY_VIDEO_LOG_FILTER.

Description

• Documented the override format in the module docstring and added support for comma-separated module=LEVEL overrides via ALWRITY_LOG_LEVEL_OVERRIDES.
• Introduced DEFAULT_NAMESPACE_LEVEL_POLICIES and a NOISY_LOGGERS list and consolidated suppression into _build_logger_level_policies() and _apply_logger_level_policies() to apply levels programmatically.
• Added parsing and deduplication logic in _parse_log_level_overrides() where the last duplicate wins and invalid entries are ignored, and unified boolean env parsing in _is_enabled() which is used in setup_clean_logging() and get_uvicorn_log_level().
• Moved the Story Video generation sink behind the feature flag ALWRITY_ENABLE_STORY_VIDEO_LOG_FILTER so it is only registered when explicitly enabled.

Testing

• Ran python -m py_compile backend/logging_config.py to verify the module compiles successfully (passed).

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Codex Task