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/alembic/versions/0008_photos_date_warning.py

@@ -0,0 +1,49 @@
+"""photos has_date_warning flag
+
+Revision ID: 0008_photos_date_warning
+Revises: 0007_folder_hidden
+Create Date: 2026-04-11
+
+Adds `photos.has_date_warning` — a denormalized boolean that's true when
+the scanner's folder/filename date guesser disagrees with the stored
+taken_at by more than 24h (or taken_at is missing and the path would
+provide a date). Surfacing this as a real column means the filter bar
+can restrict the timeline to suspicious photos without the client
+recomputing the heuristic for every row.
+
+Indexed because the filter is meant to run on top of the existing
+taken_at / folder queries that dominate the timeline, and we want the
+partial `WHERE has_date_warning` scan to stay cheap as the library
+grows.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0008_photos_date_warning"
+down_revision: Union[str, None] = "0007_folder_hidden"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "photos",
+        sa.Column(
+            "has_date_warning",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.false(),
+        ),
+    )
+    op.create_index(
+        "ix_photos_has_date_warning",
+        "photos",
+        ["has_date_warning"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_photos_has_date_warning", table_name="photos")
+    op.drop_column("photos", "has_date_warning")