sub-issue: Create mining-sources edge function (OAuth flow)

[malek10xdev]

Relates to #2750 — Phase 1 migration.

Goal

Create a new mining-sources Supabase Edge Function (Hono-based) that handles OAuth URL generation and callback for Google/Microsoft providers. Consolidate the duplicated OAuth clients currently in both the Express backend and fetch-mining-source edge function.

Endpoints

Method	Path	Purpose
POST	/:provider	Generate OAuth authorization URL with state
GET	/callback/:provider	Exchange OAuth code for tokens, upsert source

Files to create

• supabase/functions/mining-sources/index.ts — Hono app
• supabase/functions/mining-sources/oauth-handler/ — move from fetch-mining-source

Files to modify

• frontend/src/utils/oauth.ts
• frontend/src/pages/sources.vue
• frontend/src/components/campaigns/CampaignComposerDialog.vue

Files to remove

• backend/src/services/OAuth2/ (entire directory)
• backend/src/controllers/mining.controller.ts:192-241
• backend/src/routes/mining.routes.ts:40-48

Acceptance criteria

• OAuth URL generation works from edge function (no Express dependency)
• OAuth callback exchanges code, upserts source, redirects to frontend
• Frontend calls edge function directly via $saasEdgeFunctions
• All Express OAuth code removed
• Existing tests pass