feat: add topology inference database tables

Add device_role/device_role_source to devices table.
Create device_links and device_link_evidence tables for
persistent topology model.

priv/repo/migrations/20260212184424_add_device_role_to_devices.exs

@@ -0,0 +1,10 @@
+defmodule Towerops.Repo.Migrations.AddDeviceRoleToDevices do
+  use Ecto.Migration
+
+  def change do
+    alter table(:devices) do
+      add :device_role, :string
+      add :device_role_source, :string, default: "inferred"
+    end
+  end
+end

priv/repo/migrations/20260212184428_create_device_links.exs

@@ -0,0 +1,38 @@
+defmodule Towerops.Repo.Migrations.CreateDeviceLinks do
+  use Ecto.Migration
+
+  def change do
+    create table(:device_links, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+
+      add :source_device_id, references(:devices, type: :binary_id, on_delete: :delete_all),
+        null: false
+
+      add :target_device_id, references(:devices, type: :binary_id, on_delete: :delete_all)
+
+      add :source_interface_id,
+          references(:snmp_interfaces, type: :binary_id, on_delete: :nilify_all)
+
+      add :target_interface_id,
+          references(:snmp_interfaces, type: :binary_id, on_delete: :nilify_all)
+
+      add :link_type, :string, null: false
+      add :confidence, :float, null: false, default: 0.5
+      add :metadata, :map, default: %{}
+      add :discovered_remote_name, :string
+      add :discovered_remote_ip, :string
+      add :discovered_remote_mac, :string
+      add :last_confirmed_at, :utc_datetime, null: false
+      timestamps(type: :utc_datetime)
+    end
+
+    create index(:device_links, [:source_device_id])
+    create index(:device_links, [:target_device_id])
+
+    create unique_index(
+             :device_links,
+             [:source_device_id, :source_interface_id, :discovered_remote_mac],
+             name: :device_links_src_iface_remote_mac
+           )
+  end
+end

priv/repo/migrations/20260212184432_create_device_link_evidence.exs

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+defmodule Towerops.Repo.Migrations.CreateDeviceLinkEvidence do
+  use Ecto.Migration
+
+  def change do
+    create table(:device_link_evidence, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+
+      add :device_link_id, references(:device_links, type: :binary_id, on_delete: :delete_all),
+        null: false
+
+      add :evidence_type, :string, null: false
+      add :evidence_data, :map, default: %{}
+      add :observed_at, :utc_datetime, null: false
+      timestamps(type: :utc_datetime)
+    end
+
+    create index(:device_link_evidence, [:device_link_id])
+    create index(:device_link_evidence, [:observed_at])
+  end
+end