feat(date-guess): recognise YY-MM-DD HH-MM-SS Synology export filenames

The 0525.mov-style export from Synology Photos uses 2-digit years, which
the existing patterns ignored (all required \d{4}). Result: filename
gave no signal, suggestion fell through to the YYYY/MM folder layout and
snapped to day 15. The explicit HH-MM-SS half rules out random digit
triples, so we trust YY → 2000+YY for this specific shape and surface
the actual capture time, not noon.

backend/app/services/date_guess.py

@@ -64,6 +64,13 @@ def _segments(filepath: str) -> list[str]:
     return [s for s in re.split(r"[\\/]+", filepath) if s]
 
 
+# Synology Photos export: `YY-MM-DD HH-MM-SS NNNN.ext`. The explicit
+# HH-MM-SS half is what makes the 2-digit year safe to trust — a random
+# digit triple won't satisfy the hour/minute/second range checks below.
+# YY is mapped to 2000+YY (this format is a recent export convention).
+_SYNOLOGY_RE = re.compile(
+    r"(?<!\d)(\d{2})-(\d{2})-(\d{2})[\s_](\d{2})-(\d{2})-(\d{2})(?!\d)"
+)
 _COMPACT_RE = re.compile(r"(?<!\d)(\d{4})(\d{2})(\d{2})(?!\d)")
 _DASHED_RE = re.compile(r"(?<!\d)(\d{4})[-_.](\d{1,2})[-_.](\d{1,2})(?!\d)")
 _MONTH_RE = re.compile(r"(?<!\d)(\d{4})[-_.](\d{1,2})(?!\d)")
@@ -80,6 +87,24 @@ def _guess_from_string(
     if not input:
         return None
 
+    m = _SYNOLOGY_RE.search(input)
+    if m:
+        yy, mm, dd = int(m.group(1)), int(m.group(2)), int(m.group(3))
+        hh, mi, ss = int(m.group(4)), int(m.group(5)), int(m.group(6))
+        if hh < 24 and mi < 60 and ss < 60:
+            d = _make_date(2000 + yy, mm, dd)
+            if d:
+                d = d.replace(hour=hh, minute=mi, second=ss)
+                return DateGuess(
+                    date=d,
+                    confidence="high",
+                    matched=(
+                        f"{m.group(1)}-{m.group(2)}-{m.group(3)} "
+                        f"{m.group(4)}:{m.group(5)}:{m.group(6)}"
+                    ),
+                    source=source,
+                )
+
     m = _COMPACT_RE.search(input)
     if m:
         d = _make_date(int(m.group(1)), int(m.group(2)), int(m.group(3)))