feat: add a Crypto tab with live CoinGecko prices synced into Net Worth

Track individual coins (identity + quantity) in a dedicated tab, backed by
CoinGecko's Demo API (AURUM_COINGECKO_API_KEY). A holding is just another
Asset under the hood (asset_class=crypto) with its value written as a
regular AssetValuation — the existing Net Worth aggregation needs zero
changes to pick it up.

Prices never refresh in the background (no scheduler in this stack, same
reasoning as recurring transactions): a "Refresh prices" button forces a
sync, and GET /crypto/holdings lazily re-syncs at most once every 24h.
CoinGecko outages leave existing values untouched (error_key: "unreachable")
instead of failing the request. Backup/restore now carries CryptoHolding
rows too, so a restore doesn't silently strip a holding's coin identity.

Backend: 75/75 tests passing (9 new), including a real identity-map
staleness bug the tests caught in the raw-Core-upsert + ORM re-read pattern,
fixed via populate_existing=True.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

backend/alembic/versions/3e3503b61b20_add_crypto_holdings.py

@@ -0,0 +1,45 @@
+"""add crypto holdings and sync state
+
+Revision ID: 3e3503b61b20
+Revises: f6a4d8b2e017
+Create Date: 2026-08-30 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '3e3503b61b20'
+down_revision: Union[str, None] = 'f6a4d8b2e017'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'crypto_holdings',
+        sa.Column('asset_id', sa.Integer(), nullable=False),
+        sa.Column('coingecko_id', sa.String(length=100), nullable=False),
+        sa.Column('symbol', sa.String(length=20), nullable=False),
+        sa.Column('name', sa.String(length=150), nullable=False),
+        sa.Column('thumb_url', sa.String(length=500), nullable=True),
+        sa.Column('quantity', sa.Numeric(precision=38, scale=18), nullable=False),
+        sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+        sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+        sa.ForeignKeyConstraint(['asset_id'], ['assets.id'], ondelete='CASCADE'),
+        sa.PrimaryKeyConstraint('asset_id'),
+    )
+    op.create_table(
+        'crypto_sync_state',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('last_synced_at', sa.DateTime(timezone=True), nullable=True),
+        sa.PrimaryKeyConstraint('id'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('crypto_sync_state')
+    op.drop_table('crypto_holdings')