Align in_flight_scripts_json to the array shape it actually carries

Summary

• The deployment checkpoint's in_flight_scripts_json column was seeded/defaulted/documented as the legacy object form "{}", but InFlightScriptMap serializes a List<Entry> — a JSON array [{ "m": machineId, "s": stepId, "a": actionId, "t": scriptTicket }]. Parse("{}") throws (caught → treated as empty), so "{}" only ever worked via that catch fallback, and a fresh checkpoint's "{}" leaked past IsMachineBusyAsync's != "[]" pre-filter.
• Align the empty form to "[]" across all four sources of truth: entity default (+ doc-comment), EF column default, EnsureExistsAsync seed, and a DbUp migration.

Changes

• DeploymentExecutionCheckpoint.cs — default "[]" + doc-comment now describes the array shape.
• DeploymentExecutionCheckpointConfiguration.cs — HasDefaultValue("[]").
• DeploymentCheckpointService.EnsureExistsAsync — seed "[]".
• 20260615_align_inflight_scripts_default_to_array.sql — ALTER COLUMN ... SET DEFAULT '[]'::jsonb + rewrite existing empty '{}' rows to '[]' (empty object ≡ empty array — pure shape normalization).

Non-breaking

• InFlightScriptMap.Parse still tolerates any leftover legacy shape (left untouched) — existing data unaffected.
• EF always supplies an explicit value on insert, so the DB default is belt-and-braces.
• No new MachineUpgradeStatus enum value; no change to the upgrade-defer logic. Purely a data-shape alignment.

Test plan

• Integration (real Postgres, InFlightScriptStoreTests): new EnsureExists_SeedsInFlightScriptsJson_AsEmptyArray pins the seed is "[]" (discriminating vs the prior "{}"); existing suite (16) green incl. the seeded-checkpoint busy-check.
• Unit (InFlightScriptMapTests, 23): legacy "{}" / malformed / "[]" all → empty via the retained Parse catch — confirms tolerance of pre-migration rows.
• Build green; DbUp migration applies cleanly (the integration DB is freshly migrated).