feat(api,web): target_companies table, CRUD, and settings watchlist (#260)

[Taleef7]

Closes #260 (Epic #245 — Feed Engine).

Summary of Changes

1. Database Migration (014_target_companies.sql):

   • Creates target_companies table with UUID primary key, user_id, company, board_type (CHECK 'greenhouse', 'lever', 'ashby'), board_token, enabled (default true), created_at, and updated_at.
   • Adds indexes on user_id and unique index on (user_id, board_type, board_token).
   • Triggers set_updated_at() before update.

2. Backend Stores & Routing:

   • apps/api/src/data/target-company-store.postgres.ts: PostgreSQL store with strict tenant scoping (user_id = $1), listTargetCompanies, listEnabledTargetCompanies, createTargetCompany, setTargetCompanyEnabled, and deleteTargetCompany.
   • apps/api/src/data/target-company-store.ts: JSON-file store fallback facade with queued mutation concurrency safety and duplicate detection.
   • apps/api/src/routes/target-companies.ts: Endpoints GET /, POST /, PATCH /:id, DELETE /:id behind requireUser. Strict token validation /^[A-Za-z0-9._-]+$/ prevents path traversal and URL injection. Maps PG 23505 and duplicate errors to 409 Conflict.
   • Mounted at /api/target-companies in apps/api/src/app.ts.

3. Web UI & API Client:

   • apps/web/src/lib/api.ts: Exports fetchTargetCompanies, createTargetCompany, setTargetCompanyEnabled, deleteTargetCompany.
   • apps/web/src/components/target-companies.tsx: Responsive client component with 3-field add form, board type dropdown, token helper text, toggle enable button, and delete action.
   • apps/web/src/app/(app)/settings/page.tsx: Displays Target Companies watchlist section inside SectionCard.

4. Testing & Verification:

   • 16 route tests in apps/api/src/routes/target-companies.test.ts (CRUD, validation, path traversal guard foo/../bar, 409 conflict, 404, auth).
   • Store tests in apps/api/src/data/target-company-store.test.ts.
   • All 299 API tests pass. Full npm run check (lint + typecheck + Next.js build + API build) clean.

[Taleef7]

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