feat(kanban): tiempo maximo por columna con borde rojo (issue 0089)

Adds column-level max time limit. Cards whose time_in_column_ms exceeds
the limit show a red border + red halo. Columns marked as Done never
trigger the visual regardless of the limit (per spec).

Backend:
- Migration 011_column_max_time.sql adds columns.max_time_minutes
  INTEGER NOT NULL DEFAULT 0 (0 = no limit). Aditiva, idempotente.
- Column struct + ColumnPatch + UpdateColumn handle the new field;
  negatives clamp to 0; listing query includes it.
- handleUpdateColumn (PATCH /api/columns/:id) accepts max_time_minutes
  in the JSON body.

Frontend:
- Column TS interface + UpdateColumnInput updated.
- KanbanColumn context menu: new entry "Tiempo maximo" using
  window.prompt for low-friction config; shows current value when >0.
- KanbanCard receives columnOverdue prop calculated from the column
  state and card.time_in_column_ms; renders red border (var
  --mantine-color-red-6) with 2px width + 2px red halo when overdue.
- data-card-id, data-column-overdue, data-locked attributes on the card
  paper element so e2e tests / scripts can query state.

Tests: TestColumnMaxTimeMinutes_Defaults + _Update verify the schema
default, the clamp on negative input, and that updating max_time leaves
other fields untouched.

Visual regression of the red border kept out of automated e2e because
it requires either clock control or real cards aged > N minutes; will be
verified manually after merge.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/db.go

@@ -17,14 +17,15 @@ import (
 var migrationsFS embed.FS
 
 type Column struct {
-	ID        string `json:"id"`
-	Name      string `json:"name"`
-	Position  int    `json:"position"`
-	Location  string `json:"location"`
-	Width     int    `json:"width"`
-	WIPLimit  int    `json:"wip_limit"`
-	IsDone    bool   `json:"is_done"`
-	CreatedAt string `json:"created_at"`
+	ID             string `json:"id"`
+	Name           string `json:"name"`
+	Position       int    `json:"position"`
+	Location       string `json:"location"`
+	Width          int    `json:"width"`
+	WIPLimit       int    `json:"wip_limit"`
+	IsDone         bool   `json:"is_done"`
+	MaxTimeMinutes int    `json:"max_time_minutes"`
+	CreatedAt      string `json:"created_at"`
 }
 
 type Sticker struct {
@@ -305,7 +306,7 @@ func insertCardEvent(execer interface {
 // --- Columns ---
 
 func (db *DB) ListColumns() ([]Column, error) {
-	rows, err := db.conn.Query(`SELECT id, name, position, location, width, wip_limit, is_done, created_at FROM columns ORDER BY position, created_at`)
+	rows, err := db.conn.Query(`SELECT id, name, position, location, width, wip_limit, is_done, max_time_minutes, created_at FROM columns ORDER BY position, created_at`)
 	if err != nil {
 		return nil, err
 	}
@@ -314,7 +315,7 @@ func (db *DB) ListColumns() ([]Column, error) {
 	for rows.Next() {
 		var c Column
 		var isDone int
-		if err := rows.Scan(&c.ID, &c.Name, &c.Position, &c.Location, &c.Width, &c.WIPLimit, &isDone, &c.CreatedAt); err != nil {
+		if err := rows.Scan(&c.ID, &c.Name, &c.Position, &c.Location, &c.Width, &c.WIPLimit, &isDone, &c.MaxTimeMinutes, &c.CreatedAt); err != nil {
 			return nil, err
 		}
 		c.IsDone = isDone != 0
@@ -344,12 +345,13 @@ func (db *DB) CreateColumn(name string) (*Column, error) {
 }
 
 type ColumnPatch struct {
-	Name     *string
-	Position *int
-	Location *string
-	Width    *int
-	WIPLimit *int
-	IsDone   *bool
+	Name           *string
+	Position       *int
+	Location       *string
+	Width          *int
+	WIPLimit       *int
+	IsDone         *bool
+	MaxTimeMinutes *int
 }
 
 func (db *DB) UpdateColumn(id string, patch ColumnPatch) error {
@@ -411,6 +413,15 @@ func (db *DB) UpdateColumn(id string, patch ColumnPatch) error {
 			}
 		}
 	}
+	if patch.MaxTimeMinutes != nil {
+		m := *patch.MaxTimeMinutes
+		if m < 0 {
+			m = 0
+		}
+		if _, err := db.conn.Exec(`UPDATE columns SET max_time_minutes=? WHERE id=?`, m, id); err != nil {
+			return err
+		}
+	}
 	return nil
 }