Support per-keyword rank tracking intervals

[bensenescu]

Problem

Rank Tracking schedules checks at the domain/config level today. Users want to set different check intervals per keyword so high-priority terms can be tracked more often without spending credits on every keyword at the same cadence.

Evidence

• User feedback: "I like the rank tracker. I would love to see a feature added to the rank tracker through which I can set a track interval per keyword instead of per domain."

Implementation check

Checked the OpenSEO repo on main (90a04d18). Relevant files:

• src/db/app.schema.ts: scheduleInterval / nextCheckAt live on rankTrackingConfigs; rankTrackingKeywords has no schedule fields.
• src/server.ts: cron finds due configs, advances config nextCheckAt, then schedules the whole config.
• src/server/features/rank-tracking/services/RankTrackingService.ts and src/server/workflows/RankCheckWorkflow.ts: subset checks via keywordIds already exist.

Suggested direction

Keep the domain/config interval as the default. Add optional per-keyword overrides, e.g. inherit default, daily, weekly, or manual/paused.

Acceptance criteria

• Existing trackers behave the same when no keyword overrides are set.
• Users can set/reset interval overrides for individual or selected keywords.
• Scheduled checks only run keywords due under their effective interval.
• UI shows each keyword's effective interval and next scheduled check.
• Tests cover inherited intervals, overrides, due-keyword selection, and subset runs.

[mvanhorn]

Opened #40 implementing per-keyword rank-check interval overrides. Keywords can now be set to manual/daily/weekly independent of the config default, falling back to the config interval when unset; the scheduled run advances both config and per-keyword schedules and skips manual ones. Includes a migration, service + UI, and tests. Verified locally with tsc --noEmit and vitest run RankTrackingService. AI was used for assistance.