[WIP] Add subscription, net worth, and goal/debt engines

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat(analytics): subscription + net worth + goal/debt engines</issue_title>
<issue_description>## Summary
Build the remaining analytics engines for Phase 2.

Subscription Engine

• Aggregate all recurring series into subscription dashboard
• Track: monthly cost, annual cost, active/cancelled, unused (no related transactions in 90 days)
• Alert on price changes (amount variance in recurring series)

Net Worth Engine

• Sum all account balances (assets - liabilities)
• Track over time via snapshots
• Break down by account type

Goal/Debt Model

• Define financial goals (save $X by date Y, pay off debt Z)
• Track progress against goals
• Debt payoff calculator (snowball, avalanche, custom)

Acceptance

• Subscription dashboard shows all recurring items with costs
• Net worth computed correctly from account balances
• Debt payoff scenarios produce correct timelines</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat(analytics): subscription + net worth + goal/debt engines #70

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[kayodebristol]

Closing this PR to enforce the 1-PR-per-repo rule. Only one Copilot PR is allowed per repository at a time. The oldest PR (#48) will be kept active.