feat: add is_embed_enabled config to gate embed endpoints and UI

[Copilot]

Adds a new config-category boolean setting is_embed_enabled (default: 1) that acts as a master switch for the embed feature — disabling API endpoints at the HTTP layer and hiding all embed UI entry points.

Backend

• Migration: inserts is_embed_enabled BOOL into the configs table (category config, default 1)
• InitConfig: exposes the value to the frontend via the init endpoint
• EmbedController: both getAlbum and getPublicStream throw a generic 404 when the setting is off — no feature-disclosure in the response

Frontend

• lychee.d.ts: adds is_embed_enabled: boolean to InitConfig type
• LycheeState: stores and hydrates the value from the init response
• AlbumHero.vue: isEmbeddable computed returns false when disabled, hiding the embed button
• leftMenu.ts: "Embed Stream" menu item gated on is_embed_enabled

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (3d0cfac) to head (7482c27).
⚠️ Report is 1 commits behind head on master.

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