engine: minimum cooldown between rule fires — prevent the same rule running twice in a row

[hvardhan878]

Problem

The action engine polls every 10 seconds. If a rule's condition stays true (e.g. a specific URL is open), the same rule can fire again immediately after the previous execution finishes. This leads to duplicate actions — sending the same email twice, adding the same CRM entry twice, etc.

What to build

A per-rule last fired timestamp + a configurable minimum gap (default: 5 minutes). The action engine should skip a rule if now - last_fired_at < cooldown.

Two settings:

• Global default cooldown (Settings tab slider, default 5 min)
• Per-rule override (optional — could be a future issue)

Acceptance criteria

• rules table has a last_fired_at TEXT column (ISO timestamp, nullable) — add via safeAddColumn()
• Action engine updates last_fired_at on every successful fire
• Action engine skips rules where last_fired_at is within the cooldown window
• Cooldown is configurable in Settings (stored as rule_cooldown_minutes in settings table)
• Rule card in Behaviour tab shows "Last fired X ago" using the new column (already rendered if column exists)

Files to look at

• src/main/db.ts — safeAddColumn(), Rule interface, schema migrations at bottom
• src/main/actionEngine.ts — rule dispatch logic
• The column is already displayed in the rule card UI if present