feat(extension): probe shows current state before click; v1.0.6

Operator-asked 2026-05-31 (during sidecar synthetic anchor cleanup):
"the add source/subscription button idea to the firefox extension so
it can tell me if a source/artist is added or not and offer an option
to add it if it isn't." Plan tracked in Scribe task #507.

## Backend

- `ExtensionService.probe(url)` — read-only resolution. Reuses
  `_derive` for platform+slug, then 2 SELECTs. Returns one of:
  - `source_match` (exact (artist, platform, url) Source exists)
  - `artist_match` (artist exists, this URL isn't a Source yet;
     collapses the sidecar-synthetic-only case from v26.06.01.0)
  - `new` (neither exists)
  - `unknown_platform` (URL didn't match any artist-page regex)
- `GET /api/extension/probe?url=...` route with `X-Extension-Key`
  auth posture matching `/quick-add-source`. Read-only, side-effect
  free.
- 6 backend tests in tests/test_api_extension.py covering each state
  + auth + invalid URL.

## Extension

- `api.js`: `probeSource(url)` mirroring `quickAddSource` shape.
- `background.js`: `PROBE_SOURCE` + `OPEN_ARTIST_PAGE` handlers. The
  latter strips the `/api` suffix from configured `apiUrl` (placeholder
  format per options.html) and opens `${base}/artist/{slug}` in a new
  tab via `browser.tabs.create`.
- `content-script.js`: probe-first render — on page-load and SPA
  navigation, asks the backend for the URL's state and renders the
  chip in the matching color/copy on FIRST paint instead of flashing
  generic "Add" and updating after. Click handler branches:
  `source_match` → OPEN_ARTIST_PAGE; `artist_match`/`new` → existing
  ADD_AS_SOURCE flow (then re-probes so the chip flips green
  immediately, no wait for next nav).
- `content-script.css`: three state-color modifiers
  (--new, --artist-match, --source-match) on the FC parchment-on-slate
  palette. Sage for already-added, amber for artist-exists, accent
  orange for new.

## Versioning

- `extension/manifest.json` + `extension/package.json` → 1.0.6.
  build.yml's sign-extension job will fire on push to main since no
  `ext-1.0.6` Forgejo/Gitea release exists yet — exercises the
  regenerated AMO keys end-to-end.

## Behavior on the sidecar-synthetic case

Filesystem-imported "Dymkens"-style artist with only a sidecar
synthetic Source: probe returns `artist_match` (not `new`), so the
chip reads "+ Add Patreon source to Dymkens" rather than offering to
recreate the artist. Clicking adds the real Source; existing
`_source_for_sidecar` preference logic (v26.06.01.0) routes future
gallery-dl Posts to the real one.

tests/test_api_extension.py

@@ -135,6 +135,114 @@ async def test_quick_add_source_wrong_key_401(client, ext_key):
     assert resp.status_code == 401
 
 
+# --- /api/extension/probe ---------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_probe_returns_new_when_nothing_exists(client, ext_key):
+    resp = await client.get(
+        "/api/extension/probe",
+        query_string={"url": "https://www.patreon.com/freshcreator"},
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["state"] == "new"
+    assert body["platform"] == "patreon"
+    assert body["slug"] == "freshcreator"
+
+
+@pytest.mark.asyncio
+async def test_probe_returns_source_match_for_already_added(client, ext_key, db):
+    artist = Artist(name="Alice", slug="alice", is_subscription=True)
+    db.add(artist)
+    await db.flush()
+    src = Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://www.patreon.com/alice", enabled=True, config_overrides={},
+    )
+    db.add(src)
+    await db.commit()
+
+    resp = await client.get(
+        "/api/extension/probe",
+        query_string={"url": "https://www.patreon.com/alice"},
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["state"] == "source_match"
+    assert body["artist"]["slug"] == "alice"
+    assert body["source"]["url"] == "https://www.patreon.com/alice"
+    assert body["source"]["platform"] == "patreon"
+
+
+@pytest.mark.asyncio
+async def test_probe_returns_artist_match_when_only_synthetic_anchor_exists(
+    client, ext_key, db,
+):
+    """Filesystem-imported artist with only a sidecar synthetic Source
+    for the (artist, platform) — the URL the operator's browsing isn't
+    yet a real Source. The probe should collapse this into artist_match
+    so the chip says '+ Add Patreon source to Dymkens' rather than
+    '+ Add to FabledCurator' (which would re-create the artist)."""
+    artist = Artist(name="Dymkens", slug="dymkens", is_subscription=False)
+    db.add(artist)
+    await db.flush()
+    synthetic = Source(
+        artist_id=artist.id, platform="patreon",
+        url="sidecar:patreon:dymkens", enabled=False, config_overrides={},
+    )
+    db.add(synthetic)
+    await db.commit()
+
+    resp = await client.get(
+        "/api/extension/probe",
+        query_string={"url": "https://www.patreon.com/dymkens"},
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["state"] == "artist_match"
+    assert body["artist"]["slug"] == "dymkens"
+    assert "source" not in body
+
+
+@pytest.mark.asyncio
+async def test_probe_returns_unknown_platform_for_non_artist_url(client, ext_key):
+    """A patreon URL that isn't an artist page (e.g. /home, /posts/N)
+    shouldn't trigger the button. Sentinel 'unknown_platform' state
+    tells the content script to skip injection."""
+    resp = await client.get(
+        "/api/extension/probe",
+        query_string={"url": "https://www.patreon.com/posts/12345"},
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["state"] == "unknown_platform"
+
+
+@pytest.mark.asyncio
+async def test_probe_missing_key_401(client):
+    resp = await client.get(
+        "/api/extension/probe",
+        query_string={"url": "https://www.patreon.com/maewix"},
+    )
+    assert resp.status_code == 401
+
+
+@pytest.mark.asyncio
+async def test_probe_missing_url_400(client, ext_key):
+    resp = await client.get(
+        "/api/extension/probe",
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 400
+    body = await resp.get_json()
+    assert body["error"] == "invalid_body"
+
+
 @pytest.mark.asyncio
 async def test_quick_add_source_missing_body_400(client, ext_key):
     resp = await client.post(