feat: adopt sqlc for type-safe database access (Phase A Step 2)

- Add sqlc.yaml config (SQLite engine, db/queries/sqlite/, db/dbgen/ output)
- Pin sqlc v1.30.0 in sqlc.version
- Add Makefile with sqlc-install, sqlc-generate, sqlc-verify targets
- Write 14 SQL query files covering all DB domains (users, sessions,
  invites, channels, messages, reactions, voice, roles, attachments,
  admin, dm, blocks, lockouts, profile)
- Commit generated db/dbgen/ package (querier interface + typed fns)
- FTS5 search queries remain hand-written in message_queries.go;
  transactional multi-step operations unchanged in Go

Server/db/queries/sqlite/attachments.sql

@@ -0,0 +1,24 @@
+-- name: CreateAttachment :exec
+INSERT INTO attachments (id, uploader_id, filename, stored_as, mime_type, size, width, height)
+VALUES (?, ?, ?, ?, ?, ?, ?, ?);
+
+-- name: GetAttachmentByID :one
+SELECT id, message_id, filename, stored_as, mime_type, size, uploaded_at, uploader_id
+FROM attachments WHERE id = ?;
+
+-- name: GetAttachmentWithChannel :one
+SELECT a.id, a.message_id, a.filename, a.stored_as, a.mime_type, a.size,
+       a.uploaded_at, a.uploader_id, m.channel_id, c.type
+FROM attachments a
+LEFT JOIN messages m ON m.id = a.message_id
+LEFT JOIN channels c ON c.id = m.channel_id
+WHERE a.id = ?;
+
+-- name: LinkAttachmentToMessage :execresult
+UPDATE attachments SET message_id = ? WHERE id = ? AND message_id IS NULL;
+
+-- name: DeleteOrphanedAttachments :many
+DELETE FROM attachments WHERE message_id IS NULL AND uploaded_at < ? RETURNING stored_as;
+
+-- name: DeleteAttachment :exec
+DELETE FROM attachments WHERE id = ?;