Add policy-based authz and token delegation

- Replace requireAdmin (role-based) guards on all REST endpoints
  with RequirePolicy middleware backed by the existing policy engine;
  built-in admin wildcard rule (-1) preserves existing admin behaviour
  while operator rules can now grant targeted access to non-admin
  accounts (e.g. a system account allowed to list accounts)
- Wire policy engine into Server: loaded from DB at startup,
  reloaded after every policy-rule create/update/delete so changes
  take effect immediately without a server restart
- Add service_account_delegates table (migration 000008) so a human
  account can be delegated permission to issue tokens for a specific
  system account without holding the admin role
- Add token-download nonce mechanism: a short-lived (5 min),
  single-use random nonce is stored server-side after token issuance;
  the browser downloads the token as a file via
  GET /token/download/{nonce} (Content-Disposition: attachment)
  instead of copying from a flash message
- Add /service-accounts UI page for non-admin delegates
- Add TestPolicyEnforcement and TestPolicyDenyRule integration tests

Security:
- Policy engine uses deny-wins, default-deny semantics; admin wildcard
  is a compiled-in built-in and cannot be deleted via the API
- Token download nonces are 128-bit crypto/rand values, single-use,
  and expire after 5 minutes; a background goroutine evicts stale entries
- alg header validation and Ed25519 signing unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/migrations/000008_service_account_delegates.up.sql

@@ -0,0 +1,15 @@
+-- service_account_delegates tracks which human accounts are permitted to issue
+-- tokens for a given system account without holding the global admin role.
+-- Admins manage delegates; delegates can issue/rotate tokens for the specific
+-- system account only and cannot modify any other account settings.
+CREATE TABLE IF NOT EXISTS service_account_delegates (
+    id          INTEGER PRIMARY KEY,
+    account_id  INTEGER NOT NULL REFERENCES accounts(id) ON DELETE CASCADE,
+    grantee_id  INTEGER NOT NULL REFERENCES accounts(id) ON DELETE CASCADE,
+    granted_by  INTEGER REFERENCES accounts(id),
+    granted_at  TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ','now')),
+    UNIQUE (account_id, grantee_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_sa_delegates_account ON service_account_delegates (account_id);
+CREATE INDEX IF NOT EXISTS idx_sa_delegates_grantee ON service_account_delegates (grantee_id);