fix: show insights after long-running generation - #1346

Context

Insight generation can legitimately take longer than four minutes with local models and large sources. The source detail UI stopped checking the command after 120 attempts, so a completed insight remained hidden until the user navigated away and returned.

Fixes #1264

Changes

• Keep checking insight commands while their status remains non-terminal; stop on worker terminal states or three consecutive status-request failures.
• Pass an AbortSignal through status requests, cancel on unmount or replacement, and re-check cancellation after the terminal refetch.
• Perform a final insight refetch for terminal success or failure, update source query data, and surface terminal failures with a toast.
• Cover polling beyond the previous limit, terminal/error stops, sanitized status-error logging, unmount races, and backend error display with API and component tests.

User impact

Long-running insights now appear in the open Insights tab as soon as generation completes. Leaving the source stops its browser-side polling, and failed jobs no longer end silently.

Verification

• npm test — 27 test files passed, 164 tests passed.
• npm run lint — 0 errors; 7 existing warnings in unchanged files.
• npm run build — production build, TypeScript check, and static page generation passed.
• Browser reproduction at 1440×1000 with identical mocked responses — the base build stopped after 120 pending checks and continued to show the empty state; this branch completed check 121, refetched, and rendered the insight without navigation.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

How Has This Been Tested?

• Tested locally with development setup
• Added new unit tests
• Existing frontend tests pass
• Manual browser testing performed

Design Alignment

• Simplicity Over Features
• Async-First for Performance

The existing polling path remains in place; this removes its arbitrary duration limit and gives it lifecycle cancellation.

Checklist

Code Quality

• My code follows TypeScript best practices
• I have performed a self-review of the complete diff
• I have commented the lifecycle behavior where appropriate
• My changes generate no new warnings or errors

Testing

• I have added tests that prove the fix is effective
• New and existing unit tests pass locally
• I ran frontend linting
• I ran TypeScript checking through the production build

Documentation, Database, and Breaking Changes

• No documentation update is needed
• No database change or migration is needed
• No API contract or breaking change is introduced

Screenshots

Before: polling stops after 120 checks	After: the completed insight appears on check 121

Pre-Submission Verification

• I have read the contributing guide
• I have read the project vision and frontend rules
• This PR addresses approved, ready issue [Bug]: Insights tab stops polling after 4 minutes and never shows the finished insight #1264
• I have not included unrelated changes
• The PR title follows the repository's conventional commit format