feat: foundation for CRM and tax accounting specialized features

Domain Layer (SOLID Foundation):
- 5 New Entities: TaxProfile, TaxFilingSchedule, ConsultingActivity, Contract, RevenueTracking
- Client entity extended with tax-specific fields
- Multi-tenant support (company_id)

Database Migration (V015):
- Create tax_profiles table for detailed tax info
- Create tax_filing_schedules for deadline tracking
- Create consulting_activities for CRM (activity history)
- Create contracts for contract management
- Create revenue_tracking for invoice and payment tracking
- Add indexes for performance optimization

Repository Interfaces:
- ITaxProfileRepository (tax profile CRUD + risk analysis)
- ITaxFilingScheduleRepository (schedule management + deadline tracking)
- IConsultingActivityRepository (CRM activity tracking)
- IContractRepository (contract lifecycle + MRR calculation)
- IRevenueTrackingRepository (invoice + payment tracking + revenue analysis)

Architecture:
- Follows Repository Pattern with clear separation of concerns
- SOLID principles: each repo = one responsibility
- Extensible design for multi-tenant support
- Supports specialized tax accounting and CRM workflows

db/migrations/V015__AddTaxProfileAndExtendClients.sql

@@ -0,0 +1,105 @@
+-- Extend clients table with tax-specific fields
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS business_registration_number VARCHAR(20);
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS business_type VARCHAR(50);
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS establishment_date DATE;
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS annual_revenue_range VARCHAR(50);
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS employee_count INT;
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS last_tax_filing_date DATE;
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS tax_risk_level VARCHAR(20) DEFAULT 'normal';
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS next_filing_due_date DATE;
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS contact_person VARCHAR(100);
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS company_id INT REFERENCES companies(id) ON DELETE SET NULL;
+
+-- Create tax_profiles table for detailed tax information
+CREATE TABLE IF NOT EXISTS tax_profiles (
+    id                      SERIAL          PRIMARY KEY,
+    client_id               INT             NOT NULL UNIQUE REFERENCES clients(id) ON DELETE CASCADE,
+    business_registration   VARCHAR(20),
+    business_type           VARCHAR(50),
+    establishment_date      DATE,
+    annual_revenue_range    VARCHAR(50),
+    employee_count          INT,
+    accounting_method       VARCHAR(50),
+    fiscal_year_end         VARCHAR(10),
+    last_filing_date        DATE,
+    next_filing_due_date    DATE,
+    tax_risk_level          VARCHAR(20) DEFAULT 'normal',
+    previous_audit_history  BOOLEAN DEFAULT FALSE,
+    special_notes           TEXT,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Create tax_filing_schedules table for tracking schedules
+CREATE TABLE IF NOT EXISTS tax_filing_schedules (
+    id                      SERIAL          PRIMARY KEY,
+    client_id               INT             NOT NULL REFERENCES clients(id) ON DELETE CASCADE,
+    filing_type             VARCHAR(100)    NOT NULL,
+    due_date                DATE            NOT NULL,
+    filing_year             INT             NOT NULL,
+    status                  VARCHAR(50) DEFAULT 'pending',
+    assigned_to             INT             REFERENCES admin_users(id) ON DELETE SET NULL,
+    completed_date          DATE,
+    notes                   TEXT,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Create consulting_activities table for CRM (상담 활동 추적)
+CREATE TABLE IF NOT EXISTS consulting_activities (
+    id                      SERIAL          PRIMARY KEY,
+    client_id               INT             NOT NULL REFERENCES clients(id) ON DELETE CASCADE,
+    activity_type           VARCHAR(50)     NOT NULL,
+    activity_date           DATE            NOT NULL,
+    activity_time           TIME,
+    assigned_consultant     INT             REFERENCES admin_users(id) ON DELETE SET NULL,
+    description             TEXT            NOT NULL,
+    outcome                 VARCHAR(100),
+    next_followup_date      DATE,
+    notes                   TEXT,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Create contracts table (계약 관리)
+CREATE TABLE IF NOT EXISTS contracts (
+    id                      SERIAL          PRIMARY KEY,
+    client_id               INT             NOT NULL REFERENCES clients(id) ON DELETE CASCADE,
+    contract_number         VARCHAR(50)     NOT NULL UNIQUE,
+    service_type            VARCHAR(100)    NOT NULL,
+    contract_date           DATE            NOT NULL,
+    start_date              DATE            NOT NULL,
+    end_date                DATE,
+    monthly_fee             NUMERIC(10, 2),
+    total_amount            NUMERIC(10, 2),
+    payment_status          VARCHAR(50) DEFAULT 'pending',
+    status                  VARCHAR(50) DEFAULT 'active',
+    notes                   TEXT,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Create revenue_tracking table (매출 추적)
+CREATE TABLE IF NOT EXISTS revenue_tracking (
+    id                      SERIAL          PRIMARY KEY,
+    client_id               INT             NOT NULL REFERENCES clients(id) ON DELETE CASCADE,
+    invoice_number          VARCHAR(50)     NOT NULL UNIQUE,
+    invoice_date            DATE            NOT NULL,
+    service_type            VARCHAR(100),
+    amount                  NUMERIC(10, 2) NOT NULL,
+    payment_status          VARCHAR(50) DEFAULT 'pending',
+    payment_date            DATE,
+    due_date                DATE,
+    notes                   TEXT,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Create indexes for performance
+CREATE INDEX IF NOT EXISTS idx_clients_company ON clients(company_id);
+CREATE INDEX IF NOT EXISTS idx_tax_profiles_client ON tax_profiles(client_id);
+CREATE INDEX IF NOT EXISTS idx_tax_filing_schedules_client ON tax_filing_schedules(client_id);
+CREATE INDEX IF NOT EXISTS idx_tax_filing_schedules_due_date ON tax_filing_schedules(due_date);
+CREATE INDEX IF NOT EXISTS idx_consulting_activities_client ON consulting_activities(client_id);
+CREATE INDEX IF NOT EXISTS idx_contracts_client ON contracts(client_id);
+CREATE INDEX IF NOT EXISTS idx_revenue_tracking_client ON revenue_tracking(client_id);