Add multi-user auth, admin panel, and timezone support; rename to Yolkbook

- Rename app from Eggtracker to Yolkbook throughout
- Add JWT-based authentication (python-jose, passlib/bcrypt)
- Add users table; all data tables gain user_id FK for full data isolation
- Super admin credentials sourced from ADMIN_USERNAME/ADMIN_PASSWORD env vars,
  synced on every startup; orphaned rows auto-assigned to admin post-migration
- Login page with self-registration; JWT stored in localStorage (30-day expiry)
- Admin panel (/admin): list users, reset passwords, disable/enable, delete,
  and impersonate (Login As) with Return to Admin banner
- Settings modal (gear icon in nav): timezone selector and change password
- Timezone stored per-user; stats date windows computed in user's timezone;
  date input setToday() respects user timezone via Intl API
- migrate_v2.sql for existing single-user installs
- Auto-migration adds timezone column to users on startup
- Updated README with full setup, auth, admin, and migration docs

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

backend/schemas.py

@@ -4,6 +4,44 @@ from typing import Optional
 from pydantic import BaseModel, Field
 
 
+# ── Auth ──────────────────────────────────────────────────────────────────────
+
+class LoginRequest(BaseModel):
+    username: str
+    password: str
+
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+
+class ChangePasswordRequest(BaseModel):
+    current_password: str
+    new_password: str = Field(min_length=6)
+
+class ResetPasswordRequest(BaseModel):
+    new_password: str = Field(min_length=6)
+
+class TimezoneUpdate(BaseModel):
+    timezone: str = Field(min_length=1, max_length=64)
+
+
+# ── Users ─────────────────────────────────────────────────────────────────────
+
+class UserCreate(BaseModel):
+    username: str = Field(min_length=2, max_length=64)
+    password: str = Field(min_length=6)
+
+class UserOut(BaseModel):
+    id:          int
+    username:    str
+    is_admin:    bool
+    is_disabled: bool
+    timezone:    str
+    created_at:  datetime
+
+    model_config = {"from_attributes": True}
+
+
 # ── Egg Collections ───────────────────────────────────────────────────────────
 
 class EggCollectionCreate(BaseModel):