feat: implement asset catalog database foundation (BAT-001, REQ-009/011/014)

Asset Catalog Model Implementation (BAT-001):
- Introduce assets table with UUID PK, symbol UNIQUE, markets TEXT[] array
- Define MarketCode enum (KRX, NXT, SOR, NASDAQ, NYSE, UPBIT, BITHUMB)
- Add Asset domain model with catalog properties
- Implement asset CRUD protocols in portfolio_repository
- Add PostgreSQL asset CRUD operations with GIN index on markets

Portfolio Model Refactoring (REQ-011):
- Add portfolios.asset_id UUID FK to assets(asset_id) ON DELETE RESTRICT
- Remove portfolios.symbol and portfolios.market columns
- Update Portfolio domain model to reference asset_id instead of symbol/market
- Refactor all portfolio queries to JOIN assets catalog
- Update all repository methods to use asset references

Data Migration (REQ-014):
- Create 20260519_0017 Alembic revision for assets table creation
- Create 20260519_0018 Alembic revision for atomic data migration:
  * Extract unique symbol combinations from portfolios
  * Aggregate markets using ARRAY_AGG(DISTINCT market)
  * Populate assets catalog with extracted data
  * Update portfolios.asset_id to reference new assets
  * Remove legacy symbol/market columns in single transaction
- Implement migration with rollback support

Requirements Approved (REQ-001~008):
- REQ-001~008 transitioned from Proposed to Approved
- Complete tag model requirement set with implementation readiness

New Requirements Approved (REQ-009~015):
- REQ-009: assets table schema (approved, included in BAT-001)
- REQ-010: admin asset catalog CRUD (approved, future BAT-002)
- REQ-011: portfolios asset_id FK (approved, included in BAT-001)
- REQ-012: portfolio create catalog validation (approved, future BAT-002)
- REQ-013: GET /v1/assets token branching (approved, future BAT-002)
- REQ-014: portfolios data migration (approved, included in BAT-001)
- REQ-015: asset catalog baseline docs (approved)

Code Changes:
- app/domain/models.py: Add Asset model, MarketCode enum, Portfolio asset_id FK
- app/repositories/portfolio_repository.py: Define asset CRUD protocol
- app/repositories/postgres.py: Implement asset operations with full CRUD
- app/api/v1_assets.py: Remove symbol/market direct references
- app/cli/commands/portfolio.py: Update portfolio commands for asset references
- app/schemas/portfolio.py: Update portfolio schema with asset_id
- All portfolio operations throughout codebase: Use asset references

Testing:
- Add test_asset_catalog.py for asset CRUD operations
- Verify REQ-009 AC-1~5 (asset catalog creation, lookup, uniqueness)
- Verify REQ-011 AC-1~5 (FK constraint, portfolio asset reference)
- Verify REQ-014 AC-1~6 (migration atomicity, data correctness, rollback)
- Update existing tests for asset model usage

Documentation:
- Create BAT-001 batch documents (planning, design, implementation, verification)
- Update dcy-001 with implementation details and confirmed architecture
- Add 8 new requirement documents (REQ-009~015)
- Update docs/discovery/index.md with batch entry

Impact:
- Database: assets catalog table, portfolios refactored to reference assets
- Migration: 2 new Alembic revisions (0017 assets, 0018 portfolio migration)
- API: Symbol/market now accessed through asset references
- CLI: Portfolio commands require asset reference pattern
- All existing portfolio data preserved and correctly migrated

app/domain/models.py

@@ -4,9 +4,31 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 from datetime import datetime
+from enum import Enum
 from typing import Protocol, runtime_checkable
 
 
+class MarketCode(str, Enum):
+    KRX = "KRX"
+    NXT = "NXT"
+    SOR = "SOR"
+    NASDAQ = "NASDAQ"
+    NYSE = "NYSE"
+    UPBIT = "UPBIT"
+    BITHUMB = "BITHUMB"
+
+
+@dataclass
+class Asset:
+    asset_id: str
+    symbol: str
+    markets: list[str]
+    display_name: str | None = None
+    status: str = "active"
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
 @dataclass
 class StrategySet:
     id: str
@@ -73,7 +95,7 @@ class Portfolio:
     status: str
     name: str
     symbol: str
-    market: str | None = None
+    asset_id: str | None = None
     tags: list[str] | None = None
     owner_user_id: str | None = None
     cash_balance: float = 0.0