feat: initial FitCoach Pro lead management app

Next.js 16 app with full CRUD lead management:
- REST API (/api/leads) with GET (pagination), POST, PUT, DELETE (soft delete)
- Zod validation (server + client), Supabase (PostgreSQL)
- Responsive UI: leads table (desktop) + cards (mobile), modal form, toast notifications
- 43 tests (Vitest + React Testing Library): validation, components, pagination
- Docker setup (multi-stage build, health check)
- Supabase migration + 15 seed leads
- Documentation (CLAUDE.md, README, API docs)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

supabase/migrations/001_create_leads_table.sql

@@ -0,0 +1,32 @@
+-- Create leads table for FitCoach Pro lead management
+CREATE TABLE IF NOT EXISTS leads (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  first_name VARCHAR(100) NOT NULL,
+  email VARCHAR(255) NOT NULL,
+  phone VARCHAR(20),
+  fitness_goal VARCHAR(20) NOT NULL CHECK (
+    fitness_goal IN ('abnehmen', 'muskelaufbau', 'ausdauer', 'flexibilitaet')
+  ),
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  deleted_at TIMESTAMPTZ
+);
+
+-- Indexes
+CREATE UNIQUE INDEX idx_leads_email_active ON leads (email) WHERE deleted_at IS NULL;
+CREATE INDEX idx_leads_fitness_goal ON leads (fitness_goal) WHERE deleted_at IS NULL;
+CREATE INDEX idx_leads_created_at ON leads (created_at DESC) WHERE deleted_at IS NULL;
+
+-- Auto-update updated_at trigger
+CREATE OR REPLACE FUNCTION update_updated_at_column()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.updated_at = now();
+  RETURN NEW;
+END;
+$$ language 'plpgsql';
+
+CREATE TRIGGER update_leads_updated_at
+  BEFORE UPDATE ON leads
+  FOR EACH ROW
+  EXECUTE FUNCTION update_updated_at_column();