Add income tracking (Phase 1)

[derekl-beep]

Summary

Adds income tracking as a parallel feature to expenses, so pasted bank-statement credit lines (payroll, interest, cashback, e-transfers) can be logged via chat instead of only being ignored or miscategorized as expenses.

This is Phase 1: add-via-chat + read-only list. Reimbursement is a category, not a linked relationship yet — matching a reimbursement to the specific expense it repays (e.g. a friend paying back their share of a dinner you logged) is a separate Phase 2, deliberately out of scope here so this could ship as a focused, reviewable slice.

• Backend: new income table (mirrors expenses, minus split/flag/edit/delete), save_income/get_income tools, GET /income, a closed INCOME_CATEGORIES list (Salary, Interest, Rebate, Reimbursement, Transfer, Gift, Other), and SYSTEM prompt rules for classifying pasted credit/deposit lines as income vs expense.
• Frontend: an Expenses/Income segmented toggle inside the existing Expenses pane (not a third top-level tab, so it stays out of the way on mobile), a read-only income list (mobile cards + desktop table) with a running total, refetched after every chat turn so a chat-driven save_income shows up without a reload.

Review

Ran this repo's adversarial code-review pass (8 independent finder angles + verification) against both the backend and frontend diffs before committing. Confirmed and fixed:

• Id-collision risk: income.id and expenses.id are separate sequences that can collide, and nothing stopped the model from routing an income id into delete_expense/update_expense. The SYSTEM prompt now explicitly forbids this and tells the model to say edit/delete isn't supported for income yet.
• Ambiguous category example: the Reimbursement vs. Transfer prompt examples overlapped ("a roommate's share of a bill" fit both). Reworded so Reimbursement strictly requires a matching expense already logged in the tracker.
• Missing dedup guidance: the existing overlapping-screenshot dedup instruction only covered expenses; extended it to pasted income lines too, since income has no DB-level duplicate detection.
• Small duplication cleanups: extracted a shared description-capitalization helper, added the missing description trigram index on income (mirrors the existing expense index; get_income's description_contains filter needs it).
• Also fixed a gap outside the two target files: the Vite dev proxy was never updated for /income, which would have made the feature silently non-functional in local dev.

One angle (get_income duplicating get_expenses's query-builder almost line-for-line) and one (user_id injection via a hardcoded tuple in agent/main.py) were deliberately left as-is — both match a pre-existing pattern already used elsewhere in this codebase, and fixing them would mean refactoring working, already-tested expense code beyond this feature's scope.

Test plan

• uv run pytest tests/ — 130 passed
• npm run lint — clean
• npm run build — clean
• npx playwright test e2e/income.spec.js — 2/2 (desktop + mobile)
• Full regression check: npx playwright test e2e/expenses.spec.js e2e/insights.spec.js — 18/18, no regressions
• Live-verified via Playwright screenshots at 1440x900 and 390x844 (toggle, totals, empty state, category badge fallback for the new income categories)

Roadmap / future work

• Phase 2: link a Reimbursement income row to the specific expense it repays (linked_expense_id), net-reimbursed badge on the expense row, and a net cash-flow (income − expenses) summary.
• Income category badges (Salary, Rebate, etc.) currently fall back to a plain unstyled badge since CATEGORY_ICONS/CATEGORY_COLORS only cover expense categories — cosmetic, not blocking, worth a follow-up if income-specific styling is wanted.

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