Add investment accounts, securities, positions, transactions, and price snapshots

[theMattCode]

User story

As a user, I want to track investment positions and manual valuations so my dashboard can include investment account value.

Depends on

• Issue Add financial account APIs and account management UI #13
• Issue Replace transactions MVP with ledger entries for cashflow and transfers #15

Scope

• Add backend APIs for securities, investment positions, investment transactions, and price snapshots.
• Support investment transactions for buys, sells, dividends, and fees.
• Treat dividends as income cashflow.
• Treat investment fees as expense cashflow.
• Treat buys, sells, and market movement as balance/position changes, not cashflow.
• Add manual price snapshot entry.
• Add Cockpit investment UI for securities, positions, activity, and valuations.
• Keep model provider-ready for future automated quote providers.

Non-scope

• Automated quote integration.
• Tax lots and taxable gain calculations.
• Cross-currency valuation.

Acceptance criteria

• User can create a security.
• User can record position-changing investment transactions.
• User can enter manual price snapshots.
• Investment account value can be calculated from positions and latest price snapshots.
• Dividends and investment fees appear in cashflow correctly.
• Security buys/sells do not appear as expenses or income.