feat(backend): per-Reader userscript credential with UI install and rotation (#24)

Each Reader's userscript credential is derived from TOKEN_KEY, their
Discord id and a token epoch (HMAC-SHA256, hex); only its SHA-256 sits in
readers.token_sha256, so install URLs survive restarts while a database
leak yields nothing but hashes. One credential authenticates the script
download path and the API bearer header.

- internal/token: derivation + hashing; migration 0006 adds token_epoch
- seed refreshes the owner's epoch-0 hash only before first rotation
- httpmw.Auth resolves the acting Reader from the credential hash and
  stashes it in the request context; the retired API_TOKEN resolves to
  the owner until API_TOKEN_GRACE_UNTIL, logged per use, on both the
  bearer and script-download paths
- userscript handler renders the bindmounted file with the resolved
  Reader's credential substituted for __API_TOKEN__; a legacy-path
  request during grace serves the derived credential, so devices
  self-migrate on their next update poll
- web UI: Userscripts panel with session-gated install endpoints that
  render the script directly (credential never in markup, address bar
  or a redirect) and confirm-gated rotation; atomic epoch bump + hash
  rewrite in the store
- both userscripts carry __API_TOKEN__ placeholders; the committed
  global-token literal is removed (rotating at deploy retires it for
  real — it survives in git history)
- env: TOKEN_KEY required, API_TOKEN/API_TOKEN_GRACE_UNTIL retire the
  legacy credential; docs and compose updated

.env.example

@@ -1,8 +1,19 @@
 # Copy to .env and fill in. Never commit the real .env.
 
-# Long random secret shared with the userscript's API_TOKEN. Generate one:
+# Secret every Reader's userscript credential is derived from (issue #24):
+# the backend rebuilds install URLs from it, and only SHA-256 hashes of the
+# credentials ever touch the database. Generate one:
 #   openssl rand -hex 32
-API_TOKEN=changeme-generate-a-long-random-token
+TOKEN_KEY=changeme-generate-a-long-random-token
+
+# Retired global credential, kept only during the cutover window so
+# already-installed scripts keep working. Remove both it and
+# API_TOKEN_GRACE_UNTIL once the window has passed and every device has
+# reinstalled through the web UI.
+# API_TOKEN=
+# Moment the retired credential stops resolving to the owner (YYYY-MM-DD or
+# RFC3339). Enforced in code on every request; unset means it is already dead.
+# API_TOKEN_GRACE_UNTIL=2026-08-22
 
 # The owner's Discord user ID — the one Reader every bookmark belongs to
 # (seeded at startup). Discord snowflake, e.g. 1046923170000000000.