feat: FC links a conclusive pair itself instead of asking (4392)

Operator, 2026-09-24: *"I don't want this to be manual that defeats the
convenience that I'm going for."*

Confirm-only was right while every signal was circumstantial. Time proximity
and a body that mentions Discord can never be more than suggestive, so asking
was the honest response to what FC actually knew. A shared working name is
different in kind: when the creator's own name for a piece appears in exactly
these two posts and nowhere else in their library, there is nothing left for
the operator to adjudicate, and asking is a chore FC invented for them.

AUTO_LINK_FLOOR is 1.0 and sits deliberately above IDENTITY_FLOOR's 0.75. The
gap between them IS the review queue — real evidence, not certain enough for
FC to act on alone. Measured on artist 8, of 15 name-sharing pairs: 11 are
conclusive, 2 more propose, 2 fall short of both.

Three refusals, because an auto-link is FC asserting something the operator
never saw:

  * Exactly one candidate may be conclusive. Two is not a tie to be broken by
    score — one post can tease two pieces dropped separately, and then each
    drop carries a different name from the same teaser, each individually
    conclusive. Two conclusive answers to "which drop is this" means the
    question was wrong, so both queue and FC says nothing.
  * Neither end may already be claimed by an accepted link. That link is the
    operator's decision and reassigning its other end would overrule them
    silently.
  * The whole thing is one setting, defaulting on, reversible in the UI —
    an accepted link is a row they can dismiss.

`match_post` now returns (proposed, linked) so a sweep can report what it did
on its own rather than only what it queued.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_api_settings_downloader.py

@@ -103,3 +103,35 @@ async def test_a_negative_window_is_refused(client):
         "/api/settings/import", json={"download_revisit_days": -1}
     )
     assert resp.status_code == 400
+
+
+# --- the announcement matcher's own switch ---------------------------------
+
+
+@pytest.mark.asyncio
+async def test_auto_linking_is_on_by_default(client):
+    """Operator, 2026-09-24: "I don't want this to be manual that defeats the
+    convenience that I'm going for." Defaulting ON is the change of posture
+    that request asks for, so the default is what this pins."""
+    resp = await client.get("/api/settings/import")
+    assert resp.status_code == 200
+    assert (await resp.get_json())["discord_link_auto"] is True
+
+
+@pytest.mark.asyncio
+async def test_auto_linking_can_be_turned_off(client):
+    """Confirm-only has to stay reachable — linking without asking is a change
+    of posture, and not everyone wants it."""
+    resp = await client.patch(
+        "/api/settings/import", json={"discord_link_auto": False}
+    )
+    assert resp.status_code == 200
+    assert (await resp.get_json())["discord_link_auto"] is False
+
+
+@pytest.mark.asyncio
+async def test_auto_linking_refuses_a_non_boolean(client):
+    resp = await client.patch(
+        "/api/settings/import", json={"discord_link_auto": "yes"}
+    )
+    assert resp.status_code == 400