[econ] currency WIP

apps/econ/migrations/0018_room_currency_image_name.sql

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+-- Make `room_currency.image_name` nullable, and turn the empty strings already stored into
+-- nulls. Owned by the `econ` worker; 0017 created the table with `image_name TEXT NOT NULL
+-- DEFAULT ''`. Generated from src/room-currency-db.ts (ROOM_CURRENCY_SCHEMA_DDL, which mirrors
+-- the table as it stands AFTER this migration) — keep in sync.
+--
+-- The client's own model has `ImageName` as a nullable string and serves NULL for a currency
+-- with no custom coin art; an empty string is a different value to its decoder, so a currency
+-- created before this read back as having art named "". Nothing can upload coin art yet, so
+-- every row is affected and every one of them should be null.
+--
+-- A rebuild rather than an ALTER: SQLite cannot drop a NOT NULL constraint in place. This is
+-- the standard four-step swap — build the new shape, copy across, drop the old, rename — and
+-- the index goes with the dropped table, so it is recreated at the end rather than left to be
+-- noticed later by a slow query. `NULLIF(image_name, '')` is the conversion: it maps '' to
+-- NULL and leaves a real name alone, so a row that somehow already carries art keeps it.
+--
+-- Safe to re-run in spirit but not in fact — it is a migration, applied once, and the rename
+-- would fail on a second pass because `room_currency_new` no longer exists.
+CREATE TABLE room_currency_new (
+  currency_id TEXT PRIMARY KEY,
+  room_id INTEGER NOT NULL,
+  name TEXT NOT NULL,
+  description TEXT NOT NULL,
+  limit_amount INTEGER NOT NULL,
+  shape INTEGER NOT NULL,
+  color INTEGER NOT NULL,
+  image_name TEXT,
+  created_at TEXT NOT NULL,
+  modified_at TEXT NOT NULL
+  );
+
+INSERT INTO room_currency_new (
+  currency_id, room_id, name, description, limit_amount, shape, color, image_name, created_at, modified_at
+  )
+SELECT
+  currency_id, room_id, name, description, limit_amount, shape, color, NULLIF(image_name, ''), created_at, modified_at
+FROM room_currency;
+
+DROP TABLE room_currency;
+
+ALTER TABLE room_currency_new RENAME TO room_currency;
+
+-- Recreated because dropping the old table took its index with it. Same index 0017 defined:
+-- every read is "this room's currencies".
+CREATE INDEX IF NOT EXISTS idx_room_currency_room ON room_currency (room_id);