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/repositories/portfolio_repository.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from datetime import datetime
 from typing import Protocol
 
-from app.domain.models import AuthToken, BrokerCredential, Daemon, LedgerEntry, LedgerTrade, Portfolio, PortfolioComment, PortfolioTag, StrategySet, TokenPrincipal, User
+from app.domain.models import Asset, AuthToken, BrokerCredential, Daemon, LedgerEntry, LedgerTrade, Portfolio, PortfolioComment, PortfolioTag, StrategySet, TokenPrincipal, User
 
 
 class PortfolioRepository(Protocol):
@@ -136,7 +136,7 @@ class PortfolioRepository(Protocol):
         broker: str,
         status: str,
         name: str,
-        symbol: str,
+        asset_id: str,
         cash_balance: float,
         asset_quantity: float,
         asset_price: float,
@@ -145,6 +145,30 @@ class PortfolioRepository(Protocol):
         avabile_strategies: list[str] | None,
     ) -> Portfolio: ...
 
+    def get_asset_by_id(self, asset_id: str) -> Asset | None: ...
+
+    def get_asset_by_symbol(self, symbol: str) -> Asset | None: ...
+
+    def list_assets(self) -> list[Asset]: ...
+
+    def insert_asset(
+        self,
+        *,
+        symbol: str,
+        markets: list[str],
+        display_name: str | None = None,
+        status: str = "active",
+    ) -> Asset: ...
+
+    def update_asset(
+        self,
+        asset_id: str,
+        *,
+        markets: list[str] | None = None,
+        display_name: str | None = None,
+        status: str | None = None,
+    ) -> Asset: ...
+
     def update_portfolio(self, portfolio_id: str, **fields: object) -> Portfolio: ...
 
     def list_portfolios(