feat(manager-core): admin auth gate (Phase 3a)

Closes the regression risk of the admin API and dashboard being open
to anyone reaching the bound port. Required foundation before v1.1
data-plane services land.

Per-user accounts (admin_users), Argon2id passwords, env-var bootstrap
of the first admin that becomes inert once any admin exists, opaque
32-byte session token doubling as bearer credential, 24h sliding TTL
configurable via PICLOUD_SESSION_TTL_HOURS. is_active column lets
admins be deactivated without losing audit history; last-active-admin
guard on DELETE and on PATCH that flips is_active to false (sessions
also wiped on deactivation).

require_admin middleware fronts every /api/v1/admin/* route. The data
plane (/api/v1/execute/{id}), /healthz, /version, and user routes
stay open. picloud admin reset-password <username> subcommand handles
recovery without going through HTTP.

Dashboard gains /admin/login and /admin/admins surfaces, a top-bar
user menu, and a token store with a localStorage echo so refreshes
don't sign you out. Cookie-based auth works in parallel for non-SPA
clients.

Forward compatibility: future RBAC tables (admin_roles,
admin_user_roles) join on admin_users.id; the auth middleware is the
seam where role checks slot in. Email, 2FA, passkeys, and personal
API tokens are all additive without touching admin_users.

Blueprint §11.4 updated to reflect what actually shipped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -46,6 +46,18 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
+[[package]]
+name = "argon2"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c3610892ee6e0cbce8ae2700349fcf8f98adb0dbfbee85aec3c9179d29cc072"
+dependencies = [
+ "base64ct",
+ "blake2",
+ "cpufeatures",
+ "password-hash",
+]
+
 [[package]]
 name = "assert-json-diff"
 version = "2.0.2"
@@ -206,6 +218,15 @@ dependencies = [
  "serde_core",
 ]
 
+[[package]]
+name = "blake2"
+version = "0.10.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "46502ad458c9a52b69d4d4d32775c788b7a1b85e8bc9d482d92250fc0e3f8efe"
+dependencies = [
+ "digest",
+]
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -1233,6 +1254,17 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "password-hash"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "346f04948ba92c43e8469c1ee6736c7563d71012b17d40745260fe106aac2166"
+dependencies = [
+ "base64ct",
+ "rand_core 0.6.4",
+ "subtle",
+]
+
 [[package]]
 name = "pear"
 version = "0.2.9"
@@ -1337,15 +1369,20 @@ dependencies = [
 name = "picloud-manager-core"
 version = "0.5.1"
 dependencies = [
+ "argon2",
  "async-trait",
  "axum",
+ "base64",
  "chrono",
  "picloud-orchestrator-core",
  "picloud-shared",
+ "rand 0.8.6",
  "serde",
  "serde_json",
+ "sha2",
  "sqlx",
  "thiserror 1.0.69",
+ "tokio",
  "tracing",
  "url",
  "uuid",