Consolidate api::types and models type duplication

[dmwyatt]

Overview

The codebase has ~14 parallel type definitions across src/api/types.rs and src/models.rs that are nearly identical, with SQLite acting as an implicit mapping layer between them. This creates maintenance burden, bugs (see PR #124), and no meaningful abstraction.

See #126 for the full investigation and detailed type inventory.

Strategy

Three incremental PRs, each independently valuable:

1. Dead code cleanup + fix under-specified API types (size: S) — Remove always-None fields, add explicit fields to TranscriptUtterance and RecipeConfig, kill extra.get() hacks
2. Type consolidation (size: M) — Merge ~14 type pairs into one set of domain types. Cleaner after step 1 since API types will already be properly specified
3. Explicit storage layer (size: S) — Make transformations (TipTap → markdown, chat_url extraction) more visible as storage-layer concerns

Sub-issues

• #126 — Investigation (complete)
• #230 — Step 1: Clean up dead fields and fix under-specified API types
• #231 — Step 2: Consolidate to single set of domain types
• #232 — Step 3: Make storage layer transformations explicit