fix: allow free-tier LLM in dev mode, improve error visibility

From the PR description

Summary

• Free-tier LLM guard: the assertFreeTierAllowed guard was blocking all Gemini (free-tier) calls, including document-free ones like column prompt generation, because it required FREE_TIER_FIXTURE_ALLOWLIST even when no documents were involved. Fixed by returning early when documentFilenames is empty - no documents means no customer-data risk.
• Tabular prompt error: the /tabular-review/prompt route had a bare catch {} that swallowed the real exception and always returned the generic "Failed to generate prompt from LLM" message. Now logs the error server-side and includes the actual message in the 502 response.
• Upload error handling (documents.ts, projects.ts): AggregateError (which wraps multiple sub-errors) was being stringified as just the top-level message. Now expands all sub-errors and logs the full exception.

Root cause

The immediate trigger was that local dev only has GEMINI_API_KEY set, which routes to a free-tier Gemini model. The guard correctly blocked it, but the swallowed error made it impossible to diagnose without source-diving.

Test plan

• Restart backend locally with ALLOW_FREE_TIER_LLM=true in .env
• Open tabular review column editor and trigger auto-generate - should produce a prompt without error
• Verify that document-backed LLM calls (tabular generation) still enforce the allowlist when documentFilenames is non-empty

Reviewer notes

ALLOW_FREE_TIER_LLM=true must be set in .env for local dev (not committed - add to .env.example if desired). Production should leave this unset or false.

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