feat: click a coin row to view its trade history; edit transactions

Matches CoinMarketCap's flow: clicking anywhere on a coin's row in the
holdings table opens its transaction history (previously only a dedicated
icon button did). The table's own History icon is gone since the row now
does that job; the remaining "+"/delete icons get stopPropagation so they
don't also trigger the row click.

Adds the missing piece: transactions could be viewed and deleted but not
edited. New PATCH /crypto/transactions/{id} (partial update, never calls
CoinGecko, same oversell guard as creating a sell) backs a pencil button on
each history row that opens CryptoTransactionModal pre-filled — the same
dialog used for "+", now doing double duty as create/edit.

86/86 backend tests passing (4 new). Verified live against the real API:
create, edit quantity/note, 404 on unknown id, cleanup — real user data
untouched throughout.

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

backend/app/services/crypto_service.py

@@ -44,6 +44,7 @@ from app.schemas.crypto import (
     CryptoSearchResult,
     CryptoSyncResult,
     CryptoTransactionCreate,
+    CryptoTransactionUpdate,
 )
 from app.services.settings_service import get_or_create_app_settings
 
@@ -375,6 +376,44 @@ async def add_transaction(session: AsyncSession, asset_id: int, payload: CryptoT
     return _to_read(holding)
 
 
+async def update_transaction(
+    session: AsyncSession, transaction_id: int, payload: CryptoTransactionUpdate
+) -> CryptoHoldingRead:
+    """Edit an existing buy/sell entry — the "view/edit a transaction" flow
+    a plain delete-and-recreate can't offer (you'd lose the original id and
+    any history a future feature might key off it). Never calls CoinGecko,
+    same as add_transaction: value is recomputed from the last cached price."""
+    transaction = await session.get(CryptoTransaction, transaction_id)
+    if transaction is None:
+        raise HTTPException(status_code=404, detail="Crypto transaction not found")
+    asset_id = transaction.asset_id
+    holding = await _get_holding_or_404(session, asset_id)
+
+    updates = payload.model_dump(exclude_unset=True)
+    effective_type = updates.get("type", transaction.type)
+    effective_quantity = updates.get("quantity", transaction.quantity)
+
+    if effective_type == CryptoTransactionType.SELL:
+        # Same guard as add_transaction, checked against every *other*
+        # transaction on this holding — editing this one in place must not
+        # let it sell more than what the rest of the log would leave held.
+        others = [t for t in holding.transactions if t.id != transaction_id]
+        quantity_without_this, _ = _compute_position(others)
+        if effective_quantity > quantity_without_this:
+            raise HTTPException(status_code=400, detail="Cannot sell more than you currently hold")
+
+    for field, value in updates.items():
+        setattr(transaction, field, value)
+    await session.flush()
+
+    quantity, _ = _compute_position(holding.transactions)
+    if holding.last_price is not None:
+        await _upsert_valuation(session, asset_id, quantity * holding.last_price, date_.today())
+
+    await session.commit()
+    return _to_read(holding)
+
+
 async def list_transactions(session: AsyncSession, asset_id: int) -> list[CryptoTransaction]:
     await _get_holding_or_404(session, asset_id)  # 404s if the holding itself doesn't exist
     result = await session.execute(