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/migrations/versions/b2c3d4e5f6a7_add_vk_connections_table.py

@@ -0,0 +1,37 @@
+"""add vk_connections table
+
+Revision ID: b2c3d4e5f6a7
+Revises: a1b2c3d4e5f6
+Create Date: 2026-03-06 00:01:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = 'b2c3d4e5f6a7'
+down_revision = 'a1b2c3d4e5f6'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'vk_connections',
+        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column('user_id', sa.Integer(), nullable=False),
+        sa.Column('access_token', sa.Text(), nullable=False),
+        sa.Column('vk_user_id', sa.String(50), nullable=True),
+        sa.Column('first_name', sa.String(255), nullable=True),
+        sa.Column('last_name', sa.String(255), nullable=True),
+        sa.Column('is_online', sa.Boolean(), nullable=False, server_default='0'),
+        sa.Column('last_checked_at', sa.DateTime(), nullable=True),
+        sa.Column('connected_at', sa.DateTime(), nullable=False, server_default=sa.func.now()),
+        sa.Column('updated_at', sa.DateTime(), nullable=False, server_default=sa.func.now(), onupdate=sa.func.now()),
+        sa.ForeignKeyConstraint(['user_id'], ['users.id'], ondelete='CASCADE'),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('user_id'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('vk_connections')