feat: import transactions from a CSV bank statement

Fourth of the four ideas picked from the Maybe/Sure feature audit, and the
last one queued up. A 3-step wizard (account -> column mapping -> preview)
at /transactions/import, parsed entirely client-side (new lib/csv.ts —
delimiter auto-detect, 5 date formats, thousands/decimal-separator-aware
amounts) and submitted through a new all-or-nothing POST /transactions/bulk
endpoint, mirroring backup restore's validate-everything-before-touching-
anything approach.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/transaction.py

@@ -78,3 +78,16 @@ class TransactionPage(BaseModel):
     total: int
     page: int
     page_size: int
+
+
+class TransactionBulkCreate(BaseModel):
+    """CSV import (see routes/transactions.py's /bulk): the frontend parses
+    the file and maps its columns client-side, then sends already-shaped
+    rows here. All-or-nothing — same failure semantics as backup restore, so
+    a single bad row never leaves a partial import behind."""
+
+    items: list[TransactionCreate] = Field(min_length=1, max_length=5000)
+
+
+class TransactionBulkCreateResult(BaseModel):
+    created: int