PR #82 made `meta.aspect_ratio` crop Facebook (and already Instagram) feed
images at publish time, but the API and MCP surfaces only half-supported it:
you couldn't set meta at creation, the value wasn't validated, and responses
never returned it. This closes those gaps.
- New `AspectRatio` enum is the single source of truth for the allowed ratios
(1:1, 4:5, 16:9, original). App/API/MCP requests now validate via
`Rule::enum(AspectRatio::class)` — an invalid ratio is rejected everywhere
instead of silently center-cropping to square.
- API `StorePostRequest` and MCP `CreatePostTool` now accept `platforms.*.meta`;
`CreatePost` persists it. MCP create documents `meta` in its schema.
- `Api\PostPlatformResource` now exposes `meta`, so API and MCP responses return
the aspect_ratio (and other per-platform meta) a client set.
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