feat(api): auto-link rehearsal sessions on watcher upload and nc-scan

parse_rehearsal_date() extracts YYMMDD / YYYYMMDD from the file path
and get_or_create() a RehearsalSession. Both the watcher nc-upload
endpoint and the nc-scan endpoint now set song.session_id when a
dated folder is detected. Existing songs without a session_id are
back-filled on the next import of the same folder.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/src/rehearsalhub/services/session.py

@@ -0,0 +1,47 @@
+"""Helpers for parsing rehearsal session dates from Nextcloud paths."""
+
+from __future__ import annotations
+
+import re
+from datetime import date
+
+# Matches a YYMMDD or YYYYMMDD directory segment anywhere in the path.
+# e.g. "bands/my-band/231015/take.wav" → "231015" → date(2023, 10, 15)
+_YYMMDD_RE = re.compile(r"(?:^|/)(\d{6})(?:/|$)")
+_YYYYMMDD_RE = re.compile(r"(?:^|/)(\d{8})(?:/|$)")
+
+
+def parse_rehearsal_date(nc_file_path: str) -> date | None:
+    """
+    Extract a rehearsal date from a Nextcloud file path.
+
+    Supports YYMMDD (e.g. 231015 → 2023-10-15) and
+    YYYYMMDD (e.g. 20231015 → 2023-10-15) folder names.
+    Returns None if no date segment is found or the date is invalid.
+    """
+    # Try YYYYMMDD first (more specific)
+    m = _YYYYMMDD_RE.search(nc_file_path)
+    if m:
+        s = m.group(1)
+        try:
+            return date(int(s[:4]), int(s[4:6]), int(s[6:8]))
+        except ValueError:
+            pass
+
+    m = _YYMMDD_RE.search(nc_file_path)
+    if m:
+        s = m.group(1)
+        yy, mm, dd = int(s[0:2]), int(s[2:4]), int(s[4:6])
+        # Assume 2000s
+        try:
+            return date(2000 + yy, mm, dd)
+        except ValueError:
+            pass
+
+    return None
+
+
+def nc_folder_for_path(nc_file_path: str) -> str:
+    """Return the parent directory of a file path, with trailing slash."""
+    from pathlib import Path
+    return str(Path(nc_file_path).parent).rstrip("/") + "/"