Dates: true_instant on metadata responses for instant matching

The mobile viewer anchored its "messages around this photo" window on
the legacy date_taken (a wall clock stored as UTC for EXIF rows), while
SMS-API dates are true instants: the window was off by the photo's UTC
offset (4-5 h for photos taken at home; ImageApi #113 user story 3).
Verified SMS-API side: importers store backup epochs as UTC, and 40/40
sampled rows equal their source epochs exactly.

`/image/metadata` (+ GPS / date mutation responses) now carries
`true_instant` from `date_view::file_true_instant`: `row_true_instant`,
else the filename-class instant, else the earliest fs time. Server-side
for the same reason as `wall_clock`: the drain's permanent floor rows
have no `date_utc`. `set_capture_dates` fills both fields at all four
response sites.

dev/e2e: the manifest records each fixture's expected instant (the wall
read in its GPS zone, else America/New_York) and `e2e.sh check` asserts
it (97/97).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dev/e2e/gen_fixtures.py

@@ -47,6 +47,15 @@ PLACES = {
     "tokyo": (35.6595, 139.7005),
 }
 
+# IANA zone of each place — what the server's GPS rung of the offset cascade
+# resolves, so the expected true instant is the wall read in this zone.
+PLACE_ZONES = {
+    "santa_monica": "America/Los_Angeles",
+    "paris": "Europe/Paris",
+    "yosemite": "America/Los_Angeles",
+    "tokyo": "Asia/Tokyo",
+}
+
 PALETTE = [
     "0x1f4e79", "0x7a3e9d", "0x2e7d32", "0xb71c1c", "0xe65100",
     "0x006064", "0x4e342e", "0x283593", "0x827717", "0xad1457",
@@ -182,6 +191,15 @@ def expected_wall_clock(it: Item) -> str:
     return wall.strftime("%Y-%m-%dT%H:%M:%S")
 
 
+def expected_true_instant(it: Item) -> int:
+    """The true instant the server should report: the expected wall read in
+    the zone the camera was in (GPS place, else HOME_TZ — the server's
+    assumed zone). Filename walls outrank EXIF, but still take GPS evidence."""
+    wall = dt.datetime.fromisoformat(expected_wall_clock(it))
+    zone = ZoneInfo(PLACE_ZONES[it.gps]) if it.gps else HOME_TZ
+    return int(wall.replace(tzinfo=zone).timestamp())
+
+
 def _same_day(today: dt.date, year: int, hour: int) -> dt.datetime:
     # Feb 29 in a non-leap year would be invalid; fall back to Feb 28.
     try:
@@ -332,6 +350,7 @@ def main() -> int:
                 "tags": it.tags, "duplicate_of": it.copy_of,
                 # A duplicate carries its source's bytes, EXIF included.
                 "wall_clock": expected_wall_clock(by_path.get(it.copy_of, it)),
+                "true_instant": expected_true_instant(by_path.get(it.copy_of, it)),
             }
             for it in items
         ],