Address code review findings across API, web and deployment

High severity:
- wire TRUSTED_PROXY through Compose/deploy/docs and warn at web startup
- bucket public rate limits on effective client IP, add per-bookmark budget,
  and forward the signed browser identity from the share page
- disable first-run bootstrap in server deployments and document seeding
- serialize last-admin demotion in one advisory-locked transaction

Medium/low:
- cap and batch share recipient inserts
- add a shared outbound budget for preview/CardDAV/IMAP/signal work
- pin CardDAV dials to 443 and bound IMAP logout
- fix rightward kanban column drags and normalize cards on column delete
- classify backend outages in auth guards, add request timeouts and an
  error boundary, remove the dead login route, surface throttling
- enforce pinned/unpinned page reorder grouping server-side
- avoid preview work for stale bookmark versions; drop duplicate dialog fetch
- sequence-guard collection/tag/calendar/quick-find loads
- validate property config and fix MULTI_SELECT/DATE filter inputs
- document index maintenance; apply same-origin check to sign-out
- trim signup names, unique shared dates and kanban keys (migration 010),
  reset reference cache on identity changes, hide OIDC issuer, return 413
  for oversized direct bodies, escape Markdown link destinations
- remove dead template files and duplicate test helper

Verified with go build/vet, full PostgreSQL race suite, db convergence,
tsc, both ESLint configs and a Next production build.

web/app/(app)/kanban/client.tsx

@@ -49,6 +49,10 @@ export function KanbanClient() {
   const [draggedColumnId, setDraggedColumnId] = useState<string | null>(null)
   const [cardDrop, setCardDrop] = useState<CardDrop | null>(null)
   const [columnDropId, setColumnDropId] = useState<string | null>(null)
+  // Review M-4: a column drop needs a direction. "Before the target" made a
+  // rightward drag onto the adjacent column a no-op (the source was already
+  // before it), so the pointer's half of the target decides before/after.
+  const [columnDropAfter, setColumnDropAfter] = useState(false)
   const [editingId, setEditingId] = useState<string | null>(null)
   const [editLabel, setEditLabel] = useState('')
   const [adding, setAdding] = useState(false)
@@ -85,6 +89,7 @@ export function KanbanClient() {
     setDraggedColumnId(null)
     setCardDrop(null)
     setColumnDropId(null)
+    setColumnDropAfter(false)
   }, [])
 
   const finishDrag = useCallback(() => {
@@ -101,16 +106,18 @@ export function KanbanClient() {
     else { toast.error('Could not reorder columns'); load() }
   }, [load])
 
-  const reorderColumn = useCallback((sourceId: string, targetId: string) => {
+  const reorderColumn = useCallback((sourceId: string, targetId: string, afterTarget: boolean) => {
     if (sourceId === targetId) return
     const from = columns.findIndex(c => c.id === sourceId)
-    const to = columns.findIndex(c => c.id === targetId)
-    if (from < 0 || to < 0) return
+    if (from < 0 || !columns.some(c => c.id === targetId)) return
     const next = [...columns]
     const [moved] = next.splice(from, 1)
-    // Drop before the target column; this is the least surprising direction
-    // for both leftward and rightward moves.
-    next.splice(from < to ? to - 1 : to, 0, moved)
+    // The target's index after removal, then before or after it according to
+    // the pointer half the drop reported.
+    let insertAt = next.findIndex(c => c.id === targetId)
+    if (insertAt < 0) return
+    if (afterTarget) insertAt += 1
+    next.splice(insertAt, 0, moved)
     saveColumnOrder(next)
   }, [columns, saveColumnOrder])
 
@@ -223,6 +230,8 @@ export function KanbanClient() {
       if (draggedColumnId === col.id) return
       e.preventDefault()
       e.dataTransfer.dropEffect = 'move'
+      const rect = e.currentTarget.getBoundingClientRect()
+      setColumnDropAfter(e.clientX > rect.left + rect.width / 2)
       setColumnDropId(col.id)
       setCardDrop(null)
       return
@@ -237,7 +246,7 @@ export function KanbanClient() {
   const handleColumnDrop = (e: React.DragEvent<HTMLElement>, col: Column) => {
     e.preventDefault()
     if (draggedColumnId) {
-      reorderColumn(draggedColumnId, col.id)
+      reorderColumn(draggedColumnId, col.id, columnDropAfter)
       finishDrag()
       return
     }