Web app refuses to start on a database with no operations

[corentin-core]

Context

Starting the web app against a database that holds an account but no operations, budgets or planned operations kills the process: the startup lifespan computes the report, the budget forecast builds an empty frame, and pandas raises TypeError: Cannot infer number of levels from empty list when it turns the (empty) column list into a MultiIndex. Uvicorn logs Application startup failed and exits.

The startup already means to survive this — its helper is documented as "tolerating an empty database" — but it only catches BudgetForecasterError, and this failure arrives as a TypeError from pandas. The same call happens after every write, where the failure would be a 500 instead.

Found while running the app locally against a freshly created database. Not a regression: the empty-data path has presumably never been exercised, since the app is normally started on a database that already has statements imported.

Proposed solution

• Make the budget-forecast frame handle "nothing to forecast": return an empty frame with the right index/column shape instead of feeding an empty list to MultiIndex.from_tuples.
• Keep the startup catch narrow. Widening it to Exception would hide the next real failure; the analyzer is what should be total here.
• Check the other report sections for the same assumption (a non-empty index or column set) rather than fixing only the one that raised.

Acceptance criteria

• The web app starts against a database with an account and nothing else, and the forecast views render as empty rather than erroring
• Importing the first statements into that database makes the views fill in, with no restart needed
• A test covers report computation on an empty database at the service level, not only through the web routes

Related issues

• Related to Shareable demo instance (demo mode + seed showcasing the forecast angle) #329 (demo instance, which starts from a seeded database and so never hits this)