feat: deduplicate joint/shared accounts across bank connections

Problem

When a user has joint accounts that appear in multiple bank connections (e.g. a shared chequing account visible from both a personal BMO connection and a joint RBC connection), Friday currently:

• Shows the same account twice on the accounts page
• Pulls and stores transactions from both connections, double-counting everything
• Doubles the displayed balance

Solution

Detect duplicate accounts across connections and designate one as primary. Only the primary account syncs transactions and appears in the UI.

Deduplication key

Two bank_accounts rows are duplicates when they share the same:

• mask (last 4 digits, when available)
• name (account name)
• type + subtype (e.g. depository/checking)

Implementation

DB

ALTER TABLE bank_accounts ADD COLUMN is_duplicate INTEGER NOT NULL DEFAULT 0;
ALTER TABLE bank_accounts ADD COLUMN primary_account_id TEXT REFERENCES bank_accounts(id);

Sync (server/main.py)

After upserting account rows in sync(), run a deduplication pass:

• Group bank_accounts by (mask, name, type, subtype) where mask IS NOT NULL
• For groups with >1 row: keep the row with the earliest id (or most recent sync) as primary, mark others is_duplicate=1, primary_account_id=<primary>
• Skip transaction sync for is_duplicate=1 accounts

UI (ui/server.py + accounts.html)

• Filter out is_duplicate=1 accounts from the accounts page display
• Show a small note on the primary account if duplicates were detected: "Also connected via [institution]"

MCP list()

• Exclude duplicate accounts from transaction queries by default
• Add optional include_duplicates=True param for power users

Acceptance criteria

• Same account connected via two banks → only shown once in UI
• Transactions only pulled from primary account
• Balance shown once, not doubled
• Unit tests for deduplication logic

Trace the sync() flow in server/main.py and inspect how bank_accounts schema changes are handled. Then follow account queries in ui/server.py, accounts.html, and MCP list(), including existing tests around synchronization. Done means duplicate accounts are consolidated across sync, UI, transaction queries, balances, and the requested deduplication tests pass.