Fix/issues 689 690 694 695

[Zalgia-Saga]

Summary

This PR fixes the regressions covered by issues #689, #690, #694, and #695:

• empty or degenerate motif graphs now return zeroed empty results instead of producing misleading non-empty defaults
• DB connectivity failures are surfaced as unhealthy with a reason tied to the database component
• diagnostics CLI output includes valid JSON for machine consumers while preserving human-readable output in normal mode
• feature cache entries are invalidated when the active schema version changes so stale rows are not served against newer contracts

Changes

• add the no-motifs / degenerate-graph guard in detection/motif_census.py
• improve the DB health-check failure details in utils/diagnostics_checks.py
• add JSON output support for CLI diagnostics in cli/main.py
• invalidate cached feature entries when the feature schema version changes in detection/feature_cache.py
• add focused regression tests for each scenario

Validation

• pytest -q tests/test_motif_census.py tests/test_health_check.py tests/test_cli_operations.py tests/test_feature_cache.py tests/test_cache_invalidation.py

Closes #689
Closes #690
Closes #694
Closes #695

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