feat: per-photo EXIF editor (date taken + GPS coordinates)

[karlitschek]

Adds an "Edit metadata…" entry to the per-photo overflow menu. Lets the user correct the two EXIF fields users actually edit in practice: capture date (cameras with the wrong clock; scanned old photos) and GPS location (phones without GPS; corrections).

The edits don't touch the original file. They live in a new photos-app table and overlay at read time, so:

• the original on disk stays byte-identical
• each user's overrides are private (bob's edit on a shared photo doesn't change what alice sees)
• a Reset button per field falls back to the EXIF / mtime value

Backend

• New oc_photos_metadata_edits table: composite PK (user_id, file_id), NULLable taken_at / gps_lat / gps_lng. Lat/lng stored as DECIMAL(9,6) — six decimals = ~11 cm at the equator, plenty for hand-corrected locations, no float-precision drift. Index on file_id alone for the future delete listener.
• PhotoMetadataEditMapper upsert + per-file lookup + bulk lookup for a page of fileIds. Direct DB access (composite PK is incompatible with QBMapper).
• MetadataEditController exposes: GET /api/v1/metadata/{fileId} → current overrides PUT /api/v1/metadata/{fileId} → patch (omitted = unchanged, null = clear). Authorisation via userFolder->getById($fileId) so users can't probe other users' libraries.
• IndexController.composeItem now layers the override on top of the EXIF view: edited taken_at becomes both the response's top-level takenAt (timeline sort key) AND the metadata bag's photos-original_date_time (display key), so the photos UI stays internally consistent. Edited GPS replaces the EXIF GPS array entirely.
• On save the controller also pushes the resolved taken_at into oc_photos_index.taken_at for that user's row, so the timeline ORDER BY picks up the edit immediately without a reload.

Frontend

• PhotoMetadataEditService.ts thin REST wrapper. Patch shape: omitted field = leave alone, null field = clear override.
• MetadataEditDialog.vue form-based dialog with a <input type="datetime-local"> for date and two NcTextField inputs for lat/lng. Live validation (range checks fire as the user types; Save stays disabled while values are out of range). Per-field "Reset to original" buttons that clear the stored override and re-seed from the EXIF defaults the caller passed.
• PhotoActionsMenu gets a new "Edit metadata…" entry next to "View metadata", with the same icon-on-the-left layout as the other actions. Mounted lazily so the API call only fires when the user actually opens it.

Out of scope (follow-ups)

• Map picker for GPS (latitude/longitude inputs are functional but not a great UX for non-technical users). Drop-in component candidates: vue-leaflet (already a dep on the map view) wired to a draggable marker.
• Title / description / orientation fields. Those are XMP, not EXIF, and need a different storage path because NC indexes them separately.
• Bulk edit (apply the same date offset to a multi-selection, useful for scanned albums where every photo is wrong by the same amount).
• Writing the overrides back to the file's actual EXIF (would require shelling out to exiftool / ffmpeg). Current overlay approach is non-destructive — that's a feature for now, but power users may eventually want both.

[karlitschek]

@jancborchardt @AndyScherzinger @skjnldsv @artonge

[codecov]

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