Add per-sensor temperature alert thresholds

[VACInc]

What changed

• keeps the existing any-sensor temperature alert behavior as the default
• adds an individual-sensors mode with selectable probe-specific sliders and numeric inputs
• persists an optional per-probe threshold map through the API, cache, and collection schema
• evaluates immediate and multi-minute alerts against each selected probe's own threshold
• identifies the triggering probe in notifications and clarifies per-probe active alerts
• adds migration, API, evaluator, regression, schema, UI, and catalog coverage

Why

Different temperature probes have different safe operating ranges. A single shared threshold can be too noisy for one sensor while being too permissive for another.

Validation

• go test -tags=testing ./internal/alerts -run '^TestTemperatureAlertPerSensorThresholds' -count=1
• go test -tags=testing ./internal/alerts -run '^TestUserAlertsApi$' -count=1
• go test -tags=testing ./internal/alerts -run '^TestSystemAlertsOneMin$|^TestSystemAlertsTwoMin$' -count=1
• go test -tags=testing ./internal/hub -run '^TestCollectionRulesDefault$|^TestCollectionRulesShareAllSystems$' -count=1
• bunx biome check src/components/alerts/alerts-sheet.tsx src/components/active-alerts.tsx src/lib/alerts.ts src/types.d.ts --max-diagnostics=100
• bunx lingui compile
• npm run build
• codex review --uncommitted, no correctness findings

The repository's aggregate alert test run can intermittently hit the existing asynchronous System.StartUpdater teardown nil dereference. All affected and neighboring test groups pass independently above.

[VACInc]

Opened against upstream in error; this change belongs in our internal repository.