feat(allowlist): coverage projection + applied-count + post-accept projection (#7a/#7b)

Cluster B, milestone #99. Backend for the allowlist tuning dashboard.

#7a: AllowlistService.coverage(tag_id, threshold) counts distinct images with
a prediction resolving to the tag (raw_name==tag.name OR (raw_name,category) in
the tag's aliases) scoring >= threshold — the gross candidate pool, mirroring
tasks.ml._confidence_for_tag resolution. list_all now carries applied_count
(grouped image_tag count) + coverage_count (at the row's threshold). New
GET /api/tags/<id>/allowlist/coverage?threshold= for the live what-if number.

#7b: /suggestions/accept + /alias return {allowlisted, tag_id, tag_name,
projected_count} (projection at the tag's threshold) instead of 204, so the UI
can show a non-blocking 'auto-applying to ~N images' toast. Apply still runs
async via apply_allowlist_tags — projected_count is an estimate.

Tests: coverage by threshold (direct + alias-with-category), list applied vs
coverage, coverage route (explicit/default/bad threshold), accept/alias payload
(newly-allowlisted vs already-on-list).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

tests/test_api_allowlist.py

@@ -1,6 +1,6 @@
 import pytest
 
-from backend.app.models import TagAllowlist, TagKind
+from backend.app.models import ImagePrediction, ImageRecord, TagAllowlist, TagKind
 from backend.app.services.tag_service import TagService
 
 pytestmark = pytest.mark.integration
@@ -39,3 +39,50 @@ async def test_patch_rejects_out_of_range(client, db):
         f"/api/tags/{tag.id}/allowlist", json={"min_confidence": 1.5}
     )
     assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_coverage_endpoint(client, db):
+    tag = await TagService(db).find_or_create("Cover", TagKind.general)
+    db.add(TagAllowlist(tag_id=tag.id, min_confidence=0.90))
+    for i, score in enumerate((0.95, 0.60)):
+        img = ImageRecord(
+            path=f"/images/cov{i}.jpg", sha256=f"cv{i:062d}", size_bytes=1,
+            mime="image/jpeg", width=1, height=1,
+            origin="imported_filesystem", integrity_status="unknown",
+        )
+        db.add(img)
+        await db.flush()
+        db.add(ImagePrediction(
+            image_record_id=img.id, raw_name="Cover",
+            category="general", score=score,
+        ))
+    await db.commit()
+
+    # Explicit threshold.
+    resp = await client.get(
+        f"/api/tags/{tag.id}/allowlist/coverage?threshold=0.90"
+    )
+    assert resp.status_code == 200
+    assert (await resp.get_json())["count"] == 1
+    # Lower what-if threshold widens coverage.
+    resp = await client.get(
+        f"/api/tags/{tag.id}/allowlist/coverage?threshold=0.50"
+    )
+    assert (await resp.get_json())["count"] == 2
+    # No threshold → uses the stored min_confidence (0.90).
+    resp = await client.get(f"/api/tags/{tag.id}/allowlist/coverage")
+    body = await resp.get_json()
+    assert body["count"] == 1
+    assert body["threshold"] == pytest.approx(0.90)
+
+
+@pytest.mark.asyncio
+async def test_coverage_rejects_bad_threshold(client, db):
+    tag = await TagService(db).find_or_create("Cover2", TagKind.general)
+    db.add(TagAllowlist(tag_id=tag.id))
+    await db.commit()
+    resp = await client.get(
+        f"/api/tags/{tag.id}/allowlist/coverage?threshold=2.0"
+    )
+    assert resp.status_code == 400