feat(kanban): badges done/locked + drag locked en mismo column + migrations + UX stickers

- badges: locked → tiempo bloqueado; done → fecha completion + total lead time; otherwise → tiempo en columna
- locked cards: drag permitido dentro de mismo column (cross-column rejected con notification)
- card field: locked_at desde JOIN card_lock_history (open period)
- migrations: refactor a embed.FS, archivos 002-005 extraidos de ensureColumns; ensureColumns queda como backstop
- stickers UX: opacidad 1, debajo del texto, picker estable (useRef), boton entra directo a modo con 😀, popover cierra outside, cards done filter brightness
- format: formatDateTimeShort

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

migrations/002_add_stickers.sql

@@ -0,0 +1,4 @@
+-- Add stickers column to cards. Idempotent ALTER pattern in db.go ensureColumns.
+-- Stickers persist as JSON array: [{"emoji":"🔥","x":0.5,"y":0.5}, ...]
+-- x, y in [0, 1] relative to card dimensions for resize survival.
+ALTER TABLE cards ADD COLUMN stickers TEXT NOT NULL DEFAULT '[]';

migrations/003_columns_extras.sql

@@ -0,0 +1,6 @@
+-- Columnas extra de `columns` (location, width, wip_limit, is_done).
+-- Antes vivian en ensureColumns Go. Reextraidas a migration por consistencia.
+ALTER TABLE columns ADD COLUMN location  TEXT NOT NULL DEFAULT 'board';
+ALTER TABLE columns ADD COLUMN width     INTEGER NOT NULL DEFAULT 300;
+ALTER TABLE columns ADD COLUMN wip_limit INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE columns ADD COLUMN is_done   INTEGER NOT NULL DEFAULT 0;

migrations/004_cards_extras.sql

@@ -0,0 +1,9 @@
+-- Columnas extra de `cards` (color, locked, assignee_id, completed_at, deleted_at, tags).
+-- Antes vivian en ensureColumns Go. La columna stickers va aparte en 002.
+ALTER TABLE cards ADD COLUMN color        TEXT    NOT NULL DEFAULT '';
+ALTER TABLE cards ADD COLUMN locked       INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE cards ADD COLUMN assignee_id  TEXT;
+ALTER TABLE cards ADD COLUMN completed_at TEXT;
+ALTER TABLE cards ADD COLUMN deleted_at   TEXT;
+ALTER TABLE cards ADD COLUMN tags         TEXT    NOT NULL DEFAULT '[]';
+CREATE INDEX IF NOT EXISTS idx_cards_assignee ON cards(assignee_id);

migrations/005_history_actor.sql

@@ -0,0 +1,3 @@
+-- actor_id en histories (quien movió la card / quien bloqueó).
+ALTER TABLE card_column_history ADD COLUMN actor_id TEXT;
+ALTER TABLE card_lock_history   ADD COLUMN actor_id TEXT;