the ledger schema — accounts, transactions, imports

Money is a bigint of minor units and the sign carries the direction. No stored
balance, no user_id denormalised onto transactions, category as free text until
budgets need it to be a table. Not pushed to the database yet.

README.md

@@ -2,13 +2,24 @@
 
 Money, on your own machine.
 
-**This is a skeleton.** The repository, the mount prefix (`/finances`, `/api/finances`) and the permission
-key (`finances`) are real and settled. What the plugin actually does is not written yet — the two panels
-render a placeholder and the one route answers `GET /api/finances/ping`.
+**This is a skeleton with a data model.** The repository, the mount prefix (`/finances`, `/api/finances`)
+and the permission key (`finances`) are settled. `db/schema.ts` proposes the ledger — accounts,
+transactions and imports — and has **not** been pushed to the database. The two panels still render a
+placeholder and the one route answers `GET /api/finances/ping`.
 
 Published in this state on purpose: the identity of a plugin is the part that is expensive to change
 later, and the feature is the part that is cheap to add.
 
+## The ledger, in three tables
+
+- **`finances_accounts`** — what a balance is *of*. No stored balance: it is `opening_balance` plus the
+  transactions, computed on read, because a stored one drifts silently.
+- **`finances_transactions`** — the lines. Money is a `bigint` of minor units and the sign carries the
+  direction; negative leaves the account.
+- **`finances_imports`** — one row per CSV import, so a run mapped wrong can be undone in one delete
+  rather than by hand. `external_id` on a transaction makes re-importing an overlapping statement safe,
+  which is the normal case rather than the exception.
+
 ## What it ships
 
 | Part               | Present | Note                                                         |
@@ -16,7 +27,7 @@ later, and the feature is the part that is cheap to add.
 | `manifest.ts`      | yes     | identity, one permission                                     |
 | `api/router.ts`    | yes     | one route, mounted at `/api/finances`                        |
 | `web/`             | yes     | two placeholder panels and a layout                          |
-| `db/schema.ts`     | no      | no tables until the data model is settled                    |
+| `db/schema.ts`     | yes     | accounts, transactions, imports — written, NOT pushed yet     |
 | `sidecar/`         | no      | nothing long-lived yet                                       |
 | `assets/icon.png`  | no      | the dock falls back to the manifest's lucide `Wallet`        |