feat: add acoustic dedupe pipeline and docker runtime

am_deduper/acoustic.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+import json
+import subprocess
+from pathlib import Path
+
+MAX_SECONDS = 120
+
+
+def chromaprint(path: Path) -> str | None:
+    try:
+        p = subprocess.run(
+            ["fpcalc", "-length", str(MAX_SECONDS), "-json", str(path)],
+            capture_output=True, text=True, timeout=180, check=True,
+        )
+        return json.loads(p.stdout).get("fingerprint")
+    except (OSError, subprocess.SubprocessError, ValueError, json.JSONDecodeError):
+        return None
+
+
+def similarity(a: str, b: str) -> float:
+    """Conservative normalized positional similarity for equal-length prefixes."""
+    if not a or not b:
+        return 0.0
+    n = min(len(a), len(b))
+    if not n:
+        return 0.0
+    # Chromaprint JSON fingerprints are comma-separated integer features.
+    try:
+        aa, bb = [int(x) for x in a.split(",")], [int(x) for x in b.split(",")]
+    except ValueError:
+        return 0.0
+    n = min(len(aa), len(bb))
+    if not n:
+        return 0.0
+    # Feature-wise equality is intentionally conservative. A future backend
+    # can replace this with a library-specific fingerprint distance.
+    return sum(x == y for x, y in zip(aa[:n], bb[:n])) / n