feat(schema): add user preferences infrastructure for EPIC-09

[steilerDev]

Summary

• Migration 0018 (user_preferences table): INTEGER PK AUTOINCREMENT, user_id FK to users with ON DELETE CASCADE, key/value TEXT columns, UNIQUE(user_id, key) constraint, index on user_id
• Drizzle schema updated with userPreferences table definition
• Shared types added: UserPreference, PreferenceKey, DashboardCardId, UpsertPreferenceRequest, PreferencesResponse in shared/src/types/preference.ts
• Wiki updates: Architecture.md dashboard section, Schema.md deviation fixes (invoice_budget_lines ON DELETE CASCADE, migration renumbering 0017->0018)

Design Decisions

• No new ADR needed: ADR-017 (Chart Library: Recharts) already covers the chart decision
• No new dashboard data endpoint: client fetches from existing endpoints in parallel (budget overview, timeline, budget sources, subsidy programs, invoices)
• User preferences API contract already documented in wiki API-Contract.md (GET/PATCH/DELETE /api/users/me/preferences)
• DashboardCardId type enumerates the 8 dashboard cards for the dashboard.hiddenCards preference

Schema Deviation Fixes

• invoice_budget_lines: wiki said ON DELETE SET NULL for budget FKs, actual code uses ON DELETE CASCADE (required by XOR CHECK constraint, see Bug BUG: Migration 0017 ON DELETE SET NULL incompatible with XOR CHECK constraint in SQLite #611)
• user_preferences migration: renumbered from 0017 to 0018 (0017 is invoice_budget_lines)

Test plan

• Migration 0018 applies cleanly on a fresh database
• Migration 0018 applies cleanly on a database with existing data
• Shared types compile without errors
• Drizzle schema type-checks correctly

Refs #470

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 1.15.0-beta.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 1.15.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀