Add database-backed channels with role-based visibility and web dashboard

Replaces env-var channel keys with a Channel database model and periodic
DB refresh in the collector. Adds Channels dashboard page with QR codes,
channel visibility filtering on messages/dashboard APIs, and channel card
navigation to filtered messages view.

alembic/versions/20260519_2125_82dff87d6576_add_channels_table.py

@@ -0,0 +1,58 @@
+"""add channels table
+
+Revision ID: 82dff87d6576
+Revises: 20260515_1920
+Create Date: 2026-05-19 21:25:58.828179+00:00
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "82dff87d6576"
+down_revision: Union[str, None] = "20260515_1920"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table(
+        "channels",
+        sa.Column("name", sa.String(length=100), nullable=False),
+        sa.Column("key_hex", sa.String(length=64), nullable=False),
+        sa.Column("channel_hash", sa.String(length=2), nullable=False),
+        sa.Column("visibility", sa.String(length=20), nullable=False),
+        sa.Column("enabled", sa.Boolean(), nullable=False),
+        sa.Column("id", sa.String(), nullable=False),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("(CURRENT_TIMESTAMP)"),
+            nullable=False,
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("(CURRENT_TIMESTAMP)"),
+            nullable=False,
+        ),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("key_hex"),
+    )
+    with op.batch_alter_table("channels", schema=None) as batch_op:
+        batch_op.create_index(batch_op.f("ix_channels_name"), ["name"], unique=True)
+
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("channels", schema=None) as batch_op:
+        batch_op.drop_index(batch_op.f("ix_channels_name"))
+
+    op.drop_table("channels")
+    # ### end Alembic commands ###