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/check_wall_clock.py

@@ -1,9 +1,10 @@
 #!/usr/bin/env python3
-"""Regression check for the displayed capture time (ADR-0001 `wall_clock`).
+"""Regression check for capture dates (ADR-0001 `wall_clock` / `true_instant`).
 
 For every indexed fixture, the manifest's expected wall clock must equal the
 `wall_clock` the server reports on each endpoint that includes the file:
-`/image/metadata`, `/memories` (today's day span) and `/photos/gps-summary`.
+`/image/metadata`, `/memories` (today's day span) and `/photos/gps-summary`;
+and `/image/metadata`'s `true_instant` must equal the expected instant.
 One disagreement anywhere is exactly the class of bug this field exists to
 end, so any mismatch fails the run.
 
@@ -35,17 +36,21 @@ def main() -> int:
     failures = 0
     for item in items:
         path, want = item["path"], item["wall_clock"]
-        got = {"metadata": api("image/metadata?path=" + urllib.parse.quote(path)).get("wall_clock")}
+        meta = api("image/metadata?path=" + urllib.parse.quote(path))
+        got = {"metadata": meta.get("wall_clock")}
         if path in memories:
             got["memories"] = memories[path]
         if path in gps:
             got["gps-summary"] = gps[path]
         bad = {k: v for k, v in got.items() if v != want}
+        # The matching anchor (SMS / location windows) must be the true instant.
+        if meta.get("true_instant") != item["true_instant"]:
+            bad["metadata true_instant"] = (meta.get("true_instant"), "expected", item["true_instant"])
         if bad:
             failures += 1
             print(f"MISMATCH {path}: expected {want}, got {bad}")
 
-    print(f"{len(items) - failures}/{len(items)} fixtures report the expected wall clock "
+    print(f"{len(items) - failures}/{len(items)} fixtures report the expected wall clock + instant "
           f"({len(memories)} in memories, {len(gps)} in gps-summary)")
     return 1 if failures else 0