feat: Claude Code auto-detect (.mcp.json) + 20 MCP tools + health endpoint

Claude Code integration:
- .mcp.json at repo root for auto-detection (uses seed admin token)
- CLAUDE.md project instructions (commands, architecture, rules)
- Pre-seeded admin token: aos_admin_seed_token_for_claude_code_2026
  - Scopes: all read + all write + mcp:read + mcp:write
  - Works out of the box after bun run src/lib/seed.ts

5 new MCP read tools (20 total: 11 read + 9 write):
- get_buchhaltung: SKR04 Konten, Journal, Saldenliste, EÜR summary
- get_anlagen: Anlagenverzeichnis with AfA, book values, GWG
- get_kassenbuch: Cash book entries, running balance
- get_belege: Receipts with SHA-256 content hashes
- get_finanzamt: VAT returns, Jahreserklärungen, Jahresarchive

Fix: api-auth regex updated to allow underscores in tokens (aos_[a-z0-9_]+)

Health endpoint: GET /api/health (DB connection + version + uptime)

src/app/api/health/route.ts

@@ -0,0 +1,25 @@
+import { NextResponse } from 'next/server'
+import { db } from '@/lib/db'
+
+export const dynamic = 'force-dynamic'
+
+// GET /api/health — system health check
+export async function GET() {
+  try {
+    const companyCount = await db.company.count()
+
+    return NextResponse.json({
+      status: 'ok',
+      timestamp: new Date().toISOString(),
+      version: '1.0.0',
+      database: { connected: true, companyCount },
+      uptime: process.uptime ? `${Math.round(process.uptime())}s` : 'n/a',
+    })
+  } catch {
+    return NextResponse.json({
+      status: 'error',
+      timestamp: new Date().toISOString(),
+      database: { connected: false },
+    }, { status: 503 })
+  }
+}