feat: editable taken_at + folder-based date repair and filter

Lets operators fix corrupted capture dates at scale. Adds an editable
Date Taken field with a folder/filename-derived suggestion hint, a bulk
Date Taken section in the multi-select sidebar that either applies one
date to the whole selection or infers a per-photo date from each path,
a warning badge on thumbnails whose stored date disagrees with the
path, and a "Date issues" filter pill so suspicious photos can be
surfaced and fixed as a group. Edits are written back to EXIF on disk
so rescans don't clobber the fix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/exif_writer.py

@@ -0,0 +1,72 @@
+"""
+EXIF write-back helpers.
+
+The rest of the app reads EXIF at scan time and stashes the result in Postgres
+(see `services/metadata.py`). This module handles the reverse direction: when
+the user corrects a date in the UI we also rewrite the relevant EXIF tags on
+disk so a later rescan won't clobber the fix and external tools see the same
+truth the DB does.
+"""
+import asyncio
+import logging
+import subprocess
+from datetime import datetime
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+EXIFTOOL_TIMEOUT_SECONDS = 30
+
+
+class ExifWriteError(RuntimeError):
+    """Raised when exiftool fails to write tags to a file."""
+
+
+def _format_exif_dt(dt: datetime) -> str:
+    return dt.strftime("%Y:%m:%d %H:%M:%S")
+
+
+async def write_taken_at(filepath: str, dt: datetime) -> None:
+    """Rewrite DateTimeOriginal / CreateDate / ModifyDate on the file.
+
+    - ``-overwrite_original`` so we don't litter the library with
+      ``<name>_original`` sidecars.
+    - ``-P`` preserves the file's mtime so the scanner's mtime-based
+      change detection stays quiet.
+    - We set all three common date tags together because different viewers
+      read different ones; keeping them in lockstep avoids confusing
+      downstream tools and our own re-extraction pass.
+    """
+    if not Path(filepath).exists():
+        raise ExifWriteError(f"File not found: {filepath}")
+
+    stamp = _format_exif_dt(dt)
+    cmd = [
+        "exiftool",
+        "-overwrite_original",
+        "-P",
+        f"-DateTimeOriginal={stamp}",
+        f"-CreateDate={stamp}",
+        f"-ModifyDate={stamp}",
+        filepath,
+    ]
+
+    def _run() -> subprocess.CompletedProcess:
+        return subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=EXIFTOOL_TIMEOUT_SECONDS,
+        )
+
+    try:
+        result = await asyncio.to_thread(_run)
+    except subprocess.TimeoutExpired as exc:
+        raise ExifWriteError(f"exiftool timed out writing {filepath}") from exc
+    except FileNotFoundError as exc:
+        raise ExifWriteError("exiftool binary not available") from exc
+
+    if result.returncode != 0:
+        msg = (result.stderr or result.stdout or "unknown error").strip()
+        logger.warning("exiftool write failed for %s: %s", filepath, msg)
+        raise ExifWriteError(msg)