Upload-time duplicate detection, FTS5 query sanitization, release guard

- Reject duplicate uploads at the API boundary (HTTP 409) instead of
  silently skipping in the background worker. Checks both ingested
  documents and in-flight jobs via content_hash on the jobs table.
- Go client handles 409 with distinct messages for already-imported
  documents vs already-queued jobs.
- Sanitize FTS5 search queries by quoting each token to prevent syntax
  errors from special characters like ?, *, ", (), AND, OR, NOT.
- Add try/except safety net around FTS5 execute for edge cases.
- Add main branch guard to release.sh to prevent releasing from
  feature branches.
- Update specs and README to reflect new behaviour.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

release.sh

@@ -65,6 +65,13 @@ if [[ -z "$FORGE" ]]; then
     exit 1
 fi
 
+# Ensure we're on main branch
+CURRENT_BRANCH="$(git -C "$SCRIPT_DIR" rev-parse --abbrev-ref HEAD)"
+if [[ "$CURRENT_BRANCH" != "main" ]]; then
+    echo "Error: releases must be made from the main branch (currently on '$CURRENT_BRANCH')"
+    exit 1
+fi
+
 if ! command -v "$FORGE" &>/dev/null; then
     echo "Error: '$FORGE' not found in PATH"
     exit 1