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/sqlc.yaml

@@ -0,0 +1,17 @@
+version: "2"
+sql:
+  - engine: "sqlite"
+    queries: "db/queries/sqlite"
+    schema: "migrations"
+    gen:
+      go:
+        package: "dbgen"
+        out: "db/dbgen"
+        sql_package: "database/sql"
+        emit_interface: true
+        emit_pointers_for_null_types: true
+        emit_prepared_queries: false
+        emit_exact_table_names: false
+        emit_empty_slices: true
+        emit_json_tags: true
+        json_tags_case_style: "camel"