fix(ownership): sync snapshots over 10 MiB; refuse oversized publishes

On 2026-09-25 a snapshot built with --history 5 (15.3 MB) was
published, and `idx ownership sync` failed for every client:
"the response body is larger than request limit: 10485760" (ureq's
default body cap). It was republished by hand with --history 2 (8.2 MB).

- snapshot download: raise the ureq body limit to 1 GiB; the file is
  still verified against the manifest's size and SHA-256. The PDF/XLSX
  report downloads get a 256 MiB limit for the same reason.
- CLI test: sync a >10 MiB snapshot over HTTP (fails without the fix
  with the exact production error).
- build-latest-ownership-snapshot.sh: --max-snapshot-bytes (default
  10485760, the limit of released clients <= v0.2.3); an oversized
  snapshot fails the build, so the published one stays untouched.
- publish-ownership-snapshot.sh passes --max-snapshot-bytes through;
  publisher tests cover the failure and that the release is unchanged.
- docs: size table and guidance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017a5Cv5Z1NzdYKgKnTiAF4E

scripts/publish-ownership-snapshot.sh

@@ -35,6 +35,9 @@ Options:
   --history <n>          Request at least <n> previous monthly above-1% reports
                          in addition to the latest (default: 0). Fewer reports
                          are accepted when the source only has fewer months.
+  --max-snapshot-bytes <n>
+                         Passed to the builder: refuse snapshots above this
+                         size (default: 10485760, the limit of idx <= v0.2.3)
   --force                Upload even if the published snapshot is up to date
   --build                Run `cargo build` before publishing
   --keep-workdir         Keep the temp workdir created by the builder helper
@@ -49,6 +52,7 @@ RELEASE_TAG="ownership-snapshot-current"
 BUILD_FIRST="0"
 KEEP_WORKDIR="0"
 HISTORY="0"
+MAX_SNAPSHOT_BYTES=""
 FORCE="0"
 PUBLISH_WORKDIR=""
 STAGE="arguments"
@@ -109,6 +113,10 @@ while [[ $# -gt 0 ]]; do
             HISTORY="${2:-}"
             shift 2
             ;;
+        --max-snapshot-bytes)
+            MAX_SNAPSHOT_BYTES="${2:-}"
+            shift 2
+            ;;
         --force)
             FORCE="1"
             shift
@@ -393,6 +401,10 @@ build_args=(
     --history "$HISTORY"
 )
 
+if [[ -n "$MAX_SNAPSHOT_BYTES" ]]; then
+    build_args+=(--max-snapshot-bytes "$MAX_SNAPSHOT_BYTES")
+fi
+
 if [[ "$KEEP_WORKDIR" == "1" ]]; then
     build_args+=(--keep-workdir)
 fi