[Bug] refresh_connection does not use Plaid Update Mode — re-auth creates new connection instead of updating existing

What was broken

Root Cause 1: No true Plaid Update Mode

refresh_connection generated a generic new Link token by calling create_link_token() with no access_token parameter. This is identical to starting a fresh bank link — Plaid has no way to know which existing connection to re-authenticate, so the user would end up with a duplicate connection and the stale one would remain needs_reauth.

Related: issue #34 (TODO was tracked there but never resolved).

Root Cause 2: disconnect FK constraint failure

When a connection has linked bank_accounts and associated transactions / transaction_entries, deleting the bank_connections row raised a FOREIGN KEY constraint failed error because child rows were not cleaned up first. This made it impossible to remove a real connection via the MCP tool or UI without manual DB surgery.

Root Cause 3: No graceful re-auth path

There was no end-to-end flow for re-authenticating a needs_reauth connection while preserving history. The only option was disconnect (broken) + reconnect (creates duplicate).

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What was fixed

Fix 1: True Plaid Update Mode in PlaidProvider.create_link_token

Added an optional access_token parameter to PlaidProvider.create_link_token(). When provided, the link token request omits products and passes access_token — this is the documented Plaid Update Mode pattern that lets a user re-authenticate an existing item in-place.

Fix 2: refresh_connection now fetches and decrypts the stored token

refresh_connection(id) now:

1. Looks up the connection row in the DB
2. Decrypts the stored plaid_access_token_encrypted
3. Calls provider.create_link_token(access_token=<decrypted>) for true Update Mode
4. Embeds connection_id in the returned URL so /link/complete knows to update the existing row

Fix 3: complete_link Update Mode

Added optional connection_id parameter to complete_link(). When present:

• Exchanges the public token (Plaid returns the same access token after re-auth)
• Updates the existing bank_connections row (status='active', refreshed token)
• Does not insert a duplicate row

Fix 4: Cascade delete in disconnect

disconnect() now deletes child rows in FK order before removing the connection:
transaction_entries → transactions → bank_accounts → sync_cursors → bank_connections
All wrapped in a single transaction for atomicity.

Fix 5: UI flow end-to-end

• /link route now accepts optional connection_id query param
• link.html injects connection_id as a hidden form field when present
• /link/complete reads connection_id from the POST body and passes it to complete_link()

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Files changed

• server/providers/plaid.py — Update Mode support in create_link_token
• server/main.py — refresh_connection, complete_link, disconnect
• ui/server.py — /link route and /link/complete endpoint
• ui/templates/link.html — hidden connection_id field
• tests/test_bank_tools.py — new tests for Update Mode, cascade delete, and unknown-id error

All existing tests pass (pre-existing unrelated failure in test_ledger_mcp_tools.py unchanged).

Start by reading server/providers/plaid.py and the refresh_connection, complete_link, and disconnect paths in server/main.py. Then inspect ui/server.py, ui/templates/link.html, and tests/test_bank_tools.py to understand the end-to-end flow. Done means Update Mode preserves the existing connection, disconnect removes dependent rows atomically, and the named tests pass.