Remove unused code from ffmpeg-commands.ts

## Problem Statement `packages/ffmpeg/src/ffmpeg-commands.ts` contains dead code that increases maintenance burden without providing value. Analysis shows ~70 lines of unused methods and error classes. ## Solution Remove the unused code and associated imports. ## Implementation Plan ### Step 1: Remove `CouldNotEncodeVideoError` error class - Delete the error class definition (lines 31-35) - Commit: "Remove unused CouldNotEncodeVideoError" ### Step 2: Remove `CouldNotFigureOutWhichCTAToShowError` error class - Delete the error class definition (lines 67-71) - Commit: "Remove unused CouldNotFigureOutWhichCTAToShowError" ### Step 3: Remove `figureOutWhichCTAToShow` method - Delete the method from FFmpegCommandsService (lines 514-546) - Remove the commented-out call in `workflows.ts` that references it - Commit: "Remove unused figureOutWhichCTAToShow method" ### Step 4: Remove `combineAudioAndVideo` method - Delete the method from FFmpegCommandsService (lines 549-559) - Commit: "Remove unused combineAudioAndVideo method" ### Step 5: Clean up unused imports - Remove `import { openai } from "@ai-sdk/openai";` - Remove `import { generateObject } from "ai";` - Commit: "Remove unused imports from ffmpeg-commands.ts" ## Decision Document - **Modules modified:** `packages/ffmpeg/src/ffmpeg-commands.ts`, `packages/ffmpeg/src/workflows.ts` - **No interface changes:** All removed code is internal/unused - **No behavioral changes:** Removing dead code only - **CTA logic:** Currently hardcoded to `"ai"` in workflows.ts; this refactor accepts that decision ## Testing Decisions - `ffmpeg-commands.ts` is a shim over FFmpeg CLI commands; not designed for unit testing - Methods are tested indirectly via workflow integration tests - No new tests required; existing workflow tests provide coverage for remaining methods - Run `pnpm run ci` to verify no regressions ## Out of Scope - Refactoring the CTA selection logic (currently hardcoded to "ai") - Adding tests for ffmpeg-commands.ts - Other cleanup in the ffmpeg package