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/src/mimic/config.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from functools import lru_cache
+from pathlib import Path
 from typing import Literal
 
 from pydantic import Field, SecretStr, field_validator
@@ -45,6 +46,10 @@ class Settings(BaseSettings):
     template_render_timeout_ms: int = 250
     output_blob_max_bytes: int = 10 * 1024 * 1024
 
+    # D-012: two on-disk pools.
+    blob_root: Path = Path("/var/lib/mimic/blobs")
+    evidence_root: Path = Path("/var/lib/mimic/evidence")
+
     @field_validator("cors_origins", mode="before")
     @classmethod
     def _split_cors(cls, value: object) -> object:

backend/src/mimic/storage/__init__.py

@@ -0,0 +1,14 @@
+"""Local file-system pools (D-012).
+
+Two separate roots, configured via env:
+- `MIMIC_BLOB_ROOT` (default `/var/lib/mimic/blobs/`) holds C2 polling output
+  blobs, content-addressed and gzip-compressed: `<aa>/<bb>/<sha256>.gz` where
+  `aa` and `bb` are the first two byte-pairs of the hex digest.
+- `MIMIC_EVIDENCE_ROOT` (default `/var/lib/mimic/evidence/`) holds user-uploaded
+  evidence files, flat layout `<engagement_id>/<evidence_id>.<ext>`, no
+  compression.
+"""
+
+from mimic.storage.blob import blob_path, store_blob
+
+__all__ = ["blob_path", "store_blob"]

backend/src/mimic/storage/blob.py

@@ -0,0 +1,51 @@
+"""Content-addressed gzip-compressed blob store (D-012)."""
+
+from __future__ import annotations
+
+import gzip
+import hashlib
+import os
+import stat
+from pathlib import Path
+
+_SHA256_HEX_LEN = 64
+
+
+def _validate_digest(sha256_hex: str) -> str:
+    if len(sha256_hex) != _SHA256_HEX_LEN or any(
+        c not in "0123456789abcdef" for c in sha256_hex.lower()
+    ):
+        raise ValueError(f"invalid sha256 digest: {sha256_hex!r}")
+    return sha256_hex.lower()
+
+
+def blob_path(root: Path | str, sha256_hex: str) -> Path:
+    """Return the absolute path of the gzip-compressed blob `<aa>/<bb>/<digest>.gz`."""
+    digest = _validate_digest(sha256_hex)
+    return Path(root) / digest[0:2] / digest[2:4] / f"{digest}.gz"
+
+
+def store_blob(root: Path | str, data: bytes) -> tuple[str, Path]:
+    """Write `data` (gzip-compressed) under its sha256 digest path.
+
+    Idempotent: an existing path with the same digest is not overwritten.
+    Directory permissions are `0750` so only the owner and the `mimic` group
+    can read.
+    """
+    digest = hashlib.sha256(data).hexdigest()
+    target = blob_path(root, digest)
+    if target.exists():
+        return digest, target
+
+    target.parent.mkdir(parents=True, exist_ok=True)
+    # 0o750: owner full, group r-x, others none. The blob root is owned by the
+    # `mimic` system user; only the application and any explicit group member
+    # (audit / backup) get read access.
+    os.chmod(target.parent, stat.S_IRWXU | stat.S_IRGRP | stat.S_IXGRP)  # noqa: S103
+
+    tmp = target.with_suffix(target.suffix + ".tmp")
+    with gzip.open(tmp, "wb") as fh:
+        fh.write(data)
+    os.chmod(tmp, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP)
+    tmp.replace(target)
+    return digest, target