Add DB sandbox provider preflight

[never2average]

Summary

• add first-class thinwedge db-sandbox status/configure/preflight commands
• make Neon the default provider path and keep Ardent as an optional branch backend
• route provider readiness through bottom-up probe scripts with CI smoke coverage
• document the CFO/operator workflow for disposable DB state

Verification

• scripts/probes/check-db-sandbox-readiness.sh --dry-run
• scripts/probes/check-db-sandbox-readiness.sh --source-provider neon --branch-backend none --include-branch-lifecycle --dry-run
• scripts/probes/check-db-sandbox-readiness.sh --source-provider planetscale --branch-backend none --dry-run
• python3 schema smoke for db_sandbox and ardent
• git diff --check
• cargo test -p thinwedge-config deserialize_finance_sandbox_config
• cargo check -p thinwedge-cli --lib

Note: focused thinwedge-cli unit test compilation was attempted but the local machine ran out of disk while compiling dependencies; cargo check for the CLI crate succeeded.

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