feat(savings): savings goals — set targets with deadlines and track progress

Summary

Allow users to define savings goals (target amount + optional deadline) and track progress toward them via chat.

Data model

CREATE TABLE savings_goals (
    id          TEXT PRIMARY KEY,
    user_id     TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    name        TEXT NOT NULL,              -- e.g. "Emergency Fund"
    target      REAL NOT NULL,             -- e.g. 10000.00
    deadline    TEXT,                      -- ISO date, optional
    line_item_id TEXT,                     -- optional: tie to a specific savings line item
    created_at  INTEGER,
    updated_at  INTEGER
);

Goal progress calculation

• Linked to a line item: progress = cumulative savings entries for that line item
• Unlinked: progress is tracked via manual add_to_savings_goal() updates or inferred from total savings

Example

Emergency Fund
Target:   $10,000
Deadline: December 2026
Saved:    $4,200  (42%)
Monthly needed to hit target: $633
On track: ✅ (saving $750/mo avg)

Acceptance criteria

• Goals can be created, updated, and deleted
• Progress calculated automatically from linked savings line items
• "Monthly needed" field computed from remaining + deadline
• Deadline optional — shows open-ended progress when absent
• Unit tests

Part of Epic #247

Start by locating the savings and chat entry points, the savings line-item handling, and any existing database migration patterns. Use the proposed savings_goals schema and the add_to_savings_goal() path as the starting points, then add unit tests for creation, updates, deletion, linked progress, deadlines, and open-ended goals. Done means all acceptance criteria are covered by tests.