feat(02-01): add sqlc queries + citext/uuid overrides; generate bindings

- sqlc.yaml: overrides citext→string and uuid→uuid.UUID (Pattern 10, Pitfall 3)
- users.sql: InsertUser :one and GetUserByEmail :one
- sessions.sql: InsertSession :exec, GetSessionWithUser :one (expires_at > now() per D-07),
  DeleteSession :exec, DeleteSessionsByUser :exec, ExtendSession :exec
- sqlc generate produces Email string (not pgtype.Text) and uuid.UUID in bindings
- go build ./internal/db/... exits 0

backend/internal/db/queries/sessions.sql

@@ -0,0 +1,19 @@
+-- name: InsertSession :exec
+INSERT INTO sessions (id, user_id, expires_at)
+VALUES ($1, $2, $3);
+
+-- name: GetSessionWithUser :one
+SELECT s.id, s.user_id, s.created_at, s.expires_at,
+       u.id AS u_id, u.email, u.password_hash, u.created_at AS u_created_at, u.updated_at AS u_updated_at
+FROM sessions s
+JOIN users u ON u.id = s.user_id
+WHERE s.id = $1 AND s.expires_at > now();
+
+-- name: DeleteSession :exec
+DELETE FROM sessions WHERE id = $1;
+
+-- name: DeleteSessionsByUser :exec
+DELETE FROM sessions WHERE user_id = $1;
+
+-- name: ExtendSession :exec
+UPDATE sessions SET expires_at = $2 WHERE id = $1;

backend/internal/db/queries/users.sql

@@ -0,0 +1,9 @@
+-- name: InsertUser :one
+INSERT INTO users (email, password_hash)
+VALUES ($1, $2)
+RETURNING id, email, password_hash, created_at, updated_at;
+
+-- name: GetUserByEmail :one
+SELECT id, email, password_hash, created_at, updated_at
+FROM users
+WHERE email = $1;

backend/sqlc.yaml

@@ -10,3 +10,10 @@ sql:
         sql_package: "pgx/v5"
         emit_json_tags: false
         emit_interface: false
+        overrides:
+          - db_type: "citext"
+            go_type: "string"
+          - db_type: "uuid"
+            go_type:
+              import: "github.com/google/uuid"
+              type: "UUID"