feat(auth): store Subsonic API keys hashed; a new key is shown once (M462 #4983)

users.api_token held each user's apiKey in plaintext and was looked up by
equality, so a leaked row or backup handed out working keys. Migration
0063 replaces it with api_token_hash (sha256, hex), computed in place
from the existing keys so every Subsonic client keeps working.

The key can no longer be read back: GET /api/me/api-token is gone, and
POST returns the new key once. Settings shows it right after Regenerate
with a copy button and a "won't be shown again" note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/users.sql

@@ -1,5 +1,5 @@
 -- name: CreateUser :one
-INSERT INTO users (username, password_hash, api_token, is_admin, display_name)
+INSERT INTO users (username, password_hash, api_token_hash, is_admin, display_name)
 VALUES ($1, $2, $3, $4, $5)
 RETURNING *;
 
@@ -17,7 +17,7 @@ RETURNING *;
 -- and the second caller's INSERT fails with a unique violation. The
 -- caller (registration handler) can retry as a regular non-admin in
 -- that case (or surface a "username taken" error to the user).
-INSERT INTO users (username, password_hash, api_token, is_admin, display_name)
+INSERT INTO users (username, password_hash, api_token_hash, is_admin, display_name)
 VALUES (
     $1, $2, $3,
     (SELECT NOT EXISTS (SELECT 1 FROM users)),
@@ -28,8 +28,8 @@ RETURNING *;
 -- name: GetUserByUsername :one
 SELECT * FROM users WHERE username = $1;
 
--- name: GetUserByAPIToken :one
-SELECT * FROM users WHERE api_token = $1;
+-- name: GetUserByAPITokenHash :one
+SELECT * FROM users WHERE api_token_hash = $1;
 
 -- name: CountUsers :one
 SELECT count(*) FROM users;
@@ -87,7 +87,7 @@ WHERE u.id = $1;
 -- Admin-driven user creation. Distinct from CreateUser/CreateUserFirstAdminRace:
 -- the caller (an admin) supplies all five fields explicitly including is_admin,
 -- so an admin can promote on creation. Used by POST /api/admin/users.
-INSERT INTO users (username, password_hash, api_token, is_admin, display_name)
+INSERT INTO users (username, password_hash, api_token_hash, is_admin, display_name)
 VALUES ($1, $2, $3, $4, $5)
 RETURNING *;
 
@@ -141,11 +141,11 @@ UPDATE users
  WHERE id = $1
 RETURNING *;
 
--- name: RegenerateApiToken :one
+-- name: RegenerateApiToken :exec
 -- Self-service: caller wants a new API token. Used by the /settings
--- API Token card's "Regenerate" button.
-UPDATE users SET api_token = $2 WHERE id = $1
-RETURNING *;
+-- API Token card's "Regenerate" button. Only the hash is stored; the
+-- handler returns the raw key once.
+UPDATE users SET api_token_hash = $2 WHERE id = $1;
 
 -- name: GetUserByEmail :one
 -- Used by forgot-password lookup. Lowercase comparison both sides