fix: auto initialize default send balance (#985)

* fix: auto initialize default send balance

* fix: tighten send access auto init flow

* refactor: centralize send balance state

* fix: separate legacy repair from admin control in send balance

Add an `address_sender.source` column to distinguish legacy / auto /
user / admin rows. `ensureDefaultSendBalance` now only repairs rows
with `source IS NULL`, so admin-disabled and user-requested rows are
never overwritten. Admin POST writes tag `source = 'admin'`; new
auto-init inserts tag `'auto'`; `requestSendMailAccess` inserts tag
`'user'`.

Bumps DB_VERSION to v0.0.8 with the usual `PRAGMA table_info` guarded
ALTER, plus a standalone SQL patch under db/.

Adds E2E regressions: legacy repair path, admin-disabled rows stay
disabled across settings and send, send after admin deletion
auto-initializes a fresh row.

* fix: drop runtime legacy repair; backfill source='legacy' on migrate

Pre-v0.0.8 schema cannot distinguish legacy request-send-access
remnants from admin-disabled rows — both share `balance = 0,
enabled = 0`. Letting ensureDefaultSendBalance repair that shape on
upgrade could silently re-enable an admin-disabled row.

Remove the runtime repair path entirely:

- `ensureDefaultSendBalance` now uses `ON CONFLICT(address) DO NOTHING`;
  existing rows are never touched.
- The v0.0.8 migration (and the matching SQL patch) backfills every
  pre-existing row with `source = 'legacy'`, making pre-migration
  state explicitly off-limits to runtime auto-init.
- E2E: flip the legacy test to the negative direction — a
  `source='legacy'` zero-balance row stays untouched by settings
  reads and send attempts. Harden `resetSenderToLegacy` to return
  404 when `meta.changes < 1`.
- Update changelog and docs: legacy/admin-disabled rows must be
  restored manually via the admin UI.

* refactor: collapse send balance auto-init to missing-row insert

Per review feedback: the runtime guarantee we actually need is
"create an address_sender row when one is missing, leave existing
rows alone". Once `ensureDefaultSendBalance` switched to
`ON CONFLICT DO NOTHING`, the `source` column, the v0.0.8 migration,
and the `resetSenderToLegacy` test endpoint became dead weight —
the DO NOTHING path already protects admin-disabled and admin-edited
rows without any provenance metadata.

- Drop `address_sender.source` and the v0.0.8 migration; revert
  DB_VERSION to v0.0.7. No schema change ships with this PR.
- Strip the `source` field from `ensureDefaultSendBalance`,
  `requestSendMailAccess`, and the admin-update path.
- Remove the `/admin/test/reset_sender_to_legacy` test endpoint and
  its E2E helper; the negative legacy-repair test it served is no
  longer needed because the runtime no longer touches existing rows.
- E2E coverage stays focused on the three guardrails: missing-row
  auto-init, admin-disabled rows stay disabled, admin deletion
  triggers a fresh re-insert.
- Tighten changelog and docs to "auto-initialize missing rows".

* docs: align common-issues with missing-row-only auto-init

The FAQ entries for "DEFAULT_SEND_BALANCE set but still No balance"
still described the old behaviour of repairing legacy
`balance = 0 && enabled = 0` rows. Rewrite both zh and en rows to
match the current runtime: only addresses with no existing
`address_sender` row get auto-initialised; legacy, admin-disabled,
and admin-edited rows must be restored manually through the admin
console.

e2e/fixtures/test-helpers.ts

@@ -183,8 +183,9 @@ export function onMailpitMessage(
 
 /**
  * Request send mail access for an address.
- * Must be called before sending mail — creates the address_sender row
- * with the DEFAULT_SEND_BALANCE configured in the worker.
+ * Kept for backward compatibility and manual-request flows. When
+ * DEFAULT_SEND_BALANCE > 0, send balance may already be auto-initialized
+ * before this endpoint is called.
  */
 export async function requestSendAccess(
   ctx: APIRequestContext,
@@ -198,6 +199,62 @@ export async function requestSendAccess(
   }
 }
 
+/**
+ * Fetch the sender access row for an address from the admin API.
+ */
+export async function getAddressSender(
+  ctx: APIRequestContext,
+  address: string,
+  workerUrl: string = WORKER_URL
+): Promise<any> {
+  const res = await ctx.get(
+    `${workerUrl}/admin/address_sender?limit=1&offset=0&address=${encodeURIComponent(address)}`,
+  );
+  if (!res.ok()) {
+    throw new Error(`Failed to get address sender: ${res.status()} ${await res.text()}`);
+  }
+  const body = await res.json();
+  if (!Array.isArray(body.results) || body.results.length < 1) {
+    throw new Error(`address_sender row not found for ${address}`);
+  }
+  return body.results[0];
+}
+
+/**
+ * Update a sender access row through the admin API.
+ */
+export async function updateAddressSender(
+  ctx: APIRequestContext,
+  opts: {
+    address: string;
+    address_id: number;
+    balance: number;
+    enabled: boolean;
+  },
+  workerUrl: string = WORKER_URL
+): Promise<void> {
+  const res = await ctx.post(`${workerUrl}/admin/address_sender`, {
+    data: opts,
+  });
+  if (!res.ok()) {
+    throw new Error(`Failed to update address sender: ${res.status()} ${await res.text()}`);
+  }
+}
+
+/**
+ * Delete a sender access row through the admin API by its id.
+ */
+export async function deleteAddressSender(
+  ctx: APIRequestContext,
+  id: number,
+  workerUrl: string = WORKER_URL
+): Promise<void> {
+  const res = await ctx.delete(`${workerUrl}/admin/address_sender/${id}`);
+  if (!res.ok()) {
+    throw new Error(`Failed to delete address sender: ${res.status()} ${await res.text()}`);
+  }
+}
+
 /**
  * Delete a test address via its JWT.
  */