Improve rule execution scheduling

[midnightdim]

Version: 1.13.x

Description

Currently, all rules are executed every X minutes according to a single global interval.
This approach is:

• Inefficient — rules that don’t need frequent execution still run.
• Inflexible — rules that need faster checks can’t override the interval.

Proposed improvements

1. Store the last execution time (last_run_ts) for each rule.
2. Allow per-rule schedule configuration (optional, falls back to global default).
3. Calculate next execution time for each rule based on its last run + interval.
4. Replace the fixed global timer with a dynamic scheduler that wakes only for the nearest due execution.

Expected benefits

• Reduced CPU wake-ups and unnecessary processing.
• More flexible scheduling per rule.
• Scales better with a larger set of rules.

Implementation notes (high-level)

• DB schema update: add exec_interval (nullable) and last_run_ts (INTEGER) to rules table.
• Scheduler logic: compute the nearest next_run_ts and set timer accordingly.
• UI: add interval override option in rule edit form.
• Keep the global default interval in settings to use as fallback.