feat: sync IsRead from IMAP \Seen / Graph isRead

[fhalamzie]

Summary

Adds an IsRead boolean column to ArchivedEmails to track the read state synced from the mail provider.

Motivation: The archive currently has no way to distinguish read from unread emails. This blocks downstream tools (search UIs, dashboards) from showing unread badges or filtering by read state.

Changes

Schema

• New IsRead boolean NOT NULL DEFAULT false column on ArchivedEmails (idempotent DO $$ ... IF NOT EXISTS migration, house style)
• EF Core entity + DbContext Fluent config (HasDefaultValue(false))
• Model snapshot updated

IMAP sync (ImapMailSyncService)

• Batch-fetches \Seen flags per UID batch via folder.FetchAsync(batch, MessageSummaryItems.Flags | MessageSummaryItems.UniqueId) — one extra round-trip per batch, not per message
• Builds a UID → isRead map, threads it through to ArchiveEmailAsync
• Gracefully defaults to false if the flag fetch fails

Graph sync (GraphMailSyncService + GraphMailArchiver)

• "isRead" added to all 4 $select lists (primary, reduced-fields fallback, no-filter fallback, single-message enrichment)
• IsRead = message.IsRead ?? false in entity construction

EML/MBox import (MailImporter)

• Explicit IsRead = false (no reliable read state in file imports)

API + MCP

• IsRead exposed in EmailSummaryDto and EmailDetailDto via FromEntity — the REST API, MCP server, and web UI all flow through these

Search (EmailCoreService)

• Both raw-SQL SELECT branches (CTE + flat) updated to include e."IsRead"
• Reader hydration updated
• ArchiveEmailAsync gains optional bool isRead = false parameter

Backward compatibility

• Existing emails default to IsRead = false (column default). A full resync updates them going forward.
• The ArchiveEmailAsync signature change is backward-compatible (new param has a default value).
• The migration is idempotent (IF NOT EXISTS) and can be safely re-run.

Testing

• No dotnet SDK was available locally to run a build check — the maintainer's CI will validate compilation.
• The IMAP flag-fetch pattern (FetchAsync(IList<UniqueId>, MessageSummaryItems.Flags | MessageSummaryItems.UniqueId)) is standard MailKit API.
• The Graph isRead property already exists on the Microsoft.Graph.Models.Message type (used at GraphMailRestorer.cs:148).

Out of scope (potential follow-ups)

• API ?isRead=true/false filter parameter
• Restore round-tripping (GraphMailRestorer hardcodes IsRead = false, ImapMailRestorer always appends with MessageFlags.Seen)
• Web UI unread badges / filtering