feat: add VK OAuth connection with health checks

- Add VkConnection model with is_online/last_checked_at fields
- Add /vk OAuth flow (connect/callback/disconnect/page)
- Add VK entry to connections dashboard
- Extend background health checker to check VK tokens via users.get
- Add Alembic migration for vk_connections table
- Add VK_CLIENT_ID/SECRET/SCOPES/API_VERSION config settings

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

web/models.py

@@ -22,6 +22,7 @@ class User(Base):
     updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
 
     evotor_connection = relationship("EvotorConnection", back_populates="user", uselist=False)
+    vk_connection = relationship("VkConnection", back_populates="user", uselist=False)
 
 
 class EvotorConnection(Base):
@@ -38,3 +39,20 @@ class EvotorConnection(Base):
     updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
 
     user = relationship("User", back_populates="evotor_connection")
+
+
+class VkConnection(Base):
+    __tablename__ = "vk_connections"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), unique=True, nullable=False)
+    access_token = Column(Text, nullable=False)
+    vk_user_id = Column(String(50), nullable=True)
+    first_name = Column(String(255), nullable=True)
+    last_name = Column(String(255), nullable=True)
+    is_online = Column(Boolean, default=False, server_default="0", nullable=False)
+    last_checked_at = Column(DateTime, nullable=True)
+    connected_at = Column(DateTime, server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    user = relationship("User", back_populates="vk_connection")