feat(backend): add content-addressed gzip blob store (D-012)

Two on-disk pools per D-012:
- `MIMIC_BLOB_ROOT` (default `/var/lib/mimic/blobs/`) holds C2 polling
  output blobs, content-addressed gzip layout `<aa>/<bb>/<sha256>.gz`.
- `MIMIC_EVIDENCE_ROOT` (default `/var/lib/mimic/evidence/`) reserved for
  user-uploaded evidence (flat per-engagement, no compression). Wired only
  in config + .env.example here; F8 endpoint lands later.

`mimic.storage.blob`:
- `blob_path(root, sha256_hex)` validates the digest and returns the CAS
  path. Raises ValueError on a malformed digest (length != 64 or non-hex).
- `store_blob(root, data)` hashes, gzip-compresses, atomically writes to
  `<aa>/<bb>/<sha256>.gz` (0o750 dir perms, 0o640 file perms). Idempotent:
  duplicate writes leave mtime untouched.

5 new unit tests cover happy path, deduplication, idempotency, malformed
digest, and the two-byte-pair directory layout.

backend/tests/unit/test_storage_blob.py

@@ -0,0 +1,49 @@
+"""Content-addressed gzip blob store (D-012)."""
+
+from __future__ import annotations
+
+import gzip
+import hashlib
+
+import pytest
+
+from mimic.storage.blob import blob_path, store_blob
+
+
+def test_blob_path_uses_two_byte_pairs(tmp_path) -> None:
+    digest = "ab" + "cd" + "ef" * 30
+    path = blob_path(tmp_path, digest)
+    assert path == tmp_path / "ab" / "cd" / f"{digest}.gz"
+
+
+def test_blob_path_rejects_invalid_digest(tmp_path) -> None:
+    with pytest.raises(ValueError, match="invalid sha256"):
+        blob_path(tmp_path, "not-a-digest")
+
+
+def test_store_blob_writes_gzip_and_returns_digest(tmp_path) -> None:
+    payload = b"hello world\n"
+    expected = hashlib.sha256(payload).hexdigest()
+    digest, path = store_blob(tmp_path, payload)
+    assert digest == expected
+    assert path == tmp_path / expected[0:2] / expected[2:4] / f"{expected}.gz"
+    with gzip.open(path, "rb") as fh:
+        assert fh.read() == payload
+
+
+def test_store_blob_is_idempotent(tmp_path) -> None:
+    payload = b"same content"
+    digest1, path1 = store_blob(tmp_path, payload)
+    mtime_before = path1.stat().st_mtime_ns
+    digest2, path2 = store_blob(tmp_path, payload)
+    assert digest1 == digest2
+    assert path1 == path2
+    assert path2.stat().st_mtime_ns == mtime_before
+
+
+def test_store_blob_dedupes_distinct_payloads(tmp_path) -> None:
+    _, p1 = store_blob(tmp_path, b"alpha")
+    _, p2 = store_blob(tmp_path, b"beta")
+    assert p1 != p2
+    assert p1.exists()
+    assert p2.exists()