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/cli/commands/portfolio.py

@@ -145,7 +145,7 @@ def create_portfolio(
     ctx: typer.Context,
     name: str = typer.Argument(..., help="Portfolio name (user-scoped unique identifier)."),
     user: str = typer.Option(..., "--user", help="Owner username (required)."),
-    symbol: str = typer.Option(..., help="Trading symbol."),
+    asset_id: str = typer.Option(..., "--asset-id", help="Asset UUID from the asset catalog."),
     broker: str = typer.Option("kis", help="Broker code."),
     status: str = typer.Option("enabled", help="Portfolio status."),
     cash_balance: float = typer.Option(0.0, min=0.0, help="Starting cash balance."),
@@ -157,7 +157,7 @@ def create_portfolio(
     force: bool = typer.Option(False, "--force", help="Skip confirmation prompt."),
 ) -> None:
     _validate_portfolio_name(name, ctx.obj.console)
-    _confirm(force, f"Create portfolio {name!r} for symbol {symbol} (user: {user})?")
+    _confirm(force, f"Create portfolio {name!r} (asset-id: {asset_id}, user: {user})?")
 
     def _callback(service, repository, app_context):
         owner = repository.get_user_by_username(user)
@@ -166,7 +166,7 @@ def create_portfolio(
         return service.create_portfolio(
             CreatePortfolioRequest(
                 name=name,
-                symbol=symbol,
+                asset_id=asset_id,
                 broker=broker,
                 status=status,
                 cash_balance=cash_balance,
@@ -191,7 +191,6 @@ def update_portfolio(
     broker: str | None = typer.Option(None, help="Broker code."),
     status: str | None = typer.Option(None, help="Portfolio status."),
     new_name: str | None = typer.Option(None, "--new-name", help="New portfolio name (requires --force or confirm)."),
-    symbol: str | None = typer.Option(None, help="Trading symbol."),
     strategy_name: str | None = typer.Option(None, help="Strategy name."),
     strategy_params: str | None = typer.Option(None, help="Strategy params as JSON object."),
     asset_price: float | None = typer.Option(None, min=0.0, help="Current asset price."),
@@ -215,7 +214,6 @@ def update_portfolio(
                 broker=broker,
                 status=status,
                 name=new_name,
-                symbol=symbol,
                 strategy_name=strategy_name,
                 strategy_params=parsed_strategy_params,
                 asset_price=asset_price,