fix: nil pointer guards, worker slack adapter, translator logic, focus/digest wiring

[LSUDOKO]

Fixes

CRITICAL (crashes)

• AI client nil pointer — Added extractContent helper guarding resp.Choices[0] access in all 5 methods. Panics on empty AI response are now handled.
• Worker nil SlackAPI — Added workerSlackAPI adapter wrapping slack.Client to prevent panic on OpenDMChannel in digest scheduler.

HIGH (feature bugs)

• Translator — Changed early return on no @-mentions → sends translation DM to current user when ambiguous language is detected without explicit mentions.
• focus_open_thread — Wired button to sendSummaryToDM(): fetches channel history, runs AI summarization, posts to DM.
• Digest threads — Populates threads section using AI GenerateDigestContent instead of always-empty slice.
• Pre-existing bug — Added missing slackAPI argument to NewController call in cmd/api/main.go (caused compilation failure on main).

MEDIUM (polish)

• Removed unused RateLimitInfo type and time import from ai/client.go.
• Updated translator test to use mock AI server.

Verification

• All Go packages build clean
• All existing tests pass (including updated translator test)

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 4dcb3d08-d32d-416c-8c14-205ded4bf697

📥 Commits

Reviewing files that changed from the base of the PR and between 9170832 and 5124a93.

📒 Files selected for processing (6)

• api/cmd/worker/main.go
• api/internal/ai/client.go
• api/internal/features/digest.go
• api/internal/features/focusmode.go
• api/internal/features/translator.go
• api/internal/features/translator_test.go

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