feat: add parent/child (epic) support to cards

[ageorg06]

Summary

• Adds parentId column to the card table (nullable self-referencing FK) enabling parent/child card hierarchy (epics)
• Cards with children automatically become epics — no separate epic entity needed
• Includes full frontend UI: EpicsBar above board lists, epic badges on cards, sub-tasks section in card detail

Changes

Schema & Backend

• parentId column on card table with SET NULL on delete
• Parent/children Drizzle relations
• setParent, getChildren, getEpicsByBoard repository methods
• setParent and getEpics tRPC procedures
• Updated cardDetailSchema and boardDetailCardSchema to include parent/children data

Frontend

• EpicsBar — collapsible horizontal bar above board lists showing epics with progress badges (done/total) and colored progress bars
• Card badge — shows parent epic title on cards that belong to an epic
• Card detail — "Epic: [title]" link when card has a parent, "Sub-tasks" section listing children with list names

Test plan

• Create a card, set it as parent of other cards via API
• Verify EpicsBar appears above board with progress
• Verify card badges show epic name
• Verify card detail shows parent link and sub-tasks
• Verify deleting an epic orphans children (parentId set to null)
• Verify a card cannot be its own parent

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