Add content_type support for fiction vs cartoon stories

[realproject7]

Context

PlotToon will publish webtoon/cartoon episodes to PlotLink as markdown containing final cut image URLs. PlotLink currently stores storylines and plots as prose-oriented records without a content category, so the platform cannot distinguish existing text-first fiction from cartoon/webtoon stories.

Related PlotToon planning: realproject7/plottoon#49 and realproject7/plottoon#42.

Naming decision

Use fiction for the existing/default text-first story format, and cartoon for image-led webtoon/comic content.

Proposed scope

DB change

• Add content_type text NOT NULL DEFAULT 'fiction' to storylines table only
• Do NOT add to plots table — all plots inherit content type from their parent storyline
• Allowed values: fiction, cartoon
• Existing rows auto-default to fiction (no backfill needed)
• Update generated Supabase types

API changes

• /api/index/storyline: accept and validate contentType field (fiction | cartoon, default fiction)
• /api/index/plot: no content type needed (inherits from storyline)
• /api/storyline/update: allow updating content_type (admin only)
• Reject invalid content types with 400

UI changes

• Story cards: add "Cartoon" badge (similar to "AI WRITER" badge) when content_type = cartoon
• Discovery page: consider adding content type filter tab or badge
• Genre remains separate from content type (a cartoon can be Fantasy, Romance, etc.)

Acceptance criteria

• Existing stories behave as fiction with no changes
• PlotToon can publish/index a storyline with content_type = cartoon without on-chain contract changes
• Invalid content types are rejected (400)
• Story cards/detail pages display Cartoon badge when applicable
• Tests cover defaulting, valid cartoon indexing, and invalid value rejection