db: drop 10 unused legacy columns from packets table

Dead fields left over from the original parser schema: srccallsign,
dstcallsign, origpacket, header, body, alive, posambiguity,
symboltable, symbolcode, messaging. None are declared in the Packet
schema, none are read anywhere in lib/ or test/, and Packet.strip_dead_fields/1
already removed them from attrs before insert.

In environments where ConsolidatePacketsColumns (20260220230000)
already DROP/RECREATEd the table without these columns the migration
is a safety-net no-op thanks to IF EXISTS; otherwise it ALTERs the
parent partitioned table and Postgres cascades to all partitions.

priv/repo/migrations/20260421100900_drop_unused_packet_columns.exs

@@ -0,0 +1,73 @@
+defmodule Aprsme.Repo.Migrations.DropUnusedPacketColumns do
+  use Ecto.Migration
+
+  @moduledoc """
+  Drops 10 legacy parser-compat columns from the partitioned `packets` table
+  that are never read by any application code. Values are stripped from incoming
+  attrs by `Aprsme.Packet.strip_dead_fields/1` before insert, so the columns
+  have been write-only noise on the wire.
+
+  Each column is `ALTER TABLE ... DROP COLUMN IF EXISTS` on the parent
+  partitioned table; PostgreSQL cascades the drop to every child partition.
+  `IF EXISTS` keeps this migration idempotent against databases where the
+  earlier `ConsolidatePacketsColumns` (20260220230000) DROP/RECREATE already
+  removed these columns.
+
+  Columns dropped and rationale (one-liner each):
+
+  * `srccallsign` - duplicate of `sender`; never read.
+  * `dstcallsign` - duplicate of `destination`; never read.
+  * `origpacket`  - duplicate of `raw_packet`; never read.
+  * `header`      - raw packet header bytes; never read after `raw_packet`
+                    superseded it.
+  * `body`        - raw packet body bytes; never read after `raw_packet`
+                    superseded it.
+  * `alive`       - legacy parser "liveness" flag (default 1); never read.
+  * `posambiguity` - duplicate of `position_ambiguity`; never read (the
+                     canonical column lives in the `data` JSONB now).
+  * `symboltable` - duplicate of `symbol_table_id`; never read.
+  * `symbolcode`  - duplicate of `symbol_code`; never read.
+  * `messaging`   - legacy int flag duplicating `aprs_messaging` (bool);
+                    never read.
+
+  The `down/0` recreates the columns with their original types from
+  `PartitionPacketsTable` (20260220200000) so the migration is mechanically
+  reversible — though column DATA is permanently lost on drop.
+  """
+
+  @dropped_columns [
+    {:srccallsign, :varchar},
+    {:dstcallsign, :varchar},
+    {:origpacket, :text},
+    {:header, :text},
+    {:body, :text},
+    {:alive, :integer, "DEFAULT 1"},
+    {:posambiguity, :integer},
+    {:symboltable, :varchar},
+    {:symbolcode, :varchar},
+    {:messaging, :integer}
+  ]
+
+  def up do
+    execute("SET LOCAL statement_timeout = '0'")
+
+    for col <- @dropped_columns do
+      name = elem(col, 0)
+      execute("ALTER TABLE packets DROP COLUMN IF EXISTS #{name}")
+    end
+  end
+
+  def down do
+    execute("SET LOCAL statement_timeout = '0'")
+
+    for col <- @dropped_columns do
+      {name, type, default} =
+        case col do
+          {n, t} -> {n, t, ""}
+          {n, t, d} -> {n, t, " " <> d}
+        end
+
+      execute("ALTER TABLE packets ADD COLUMN IF NOT EXISTS #{name} #{type}#{default}")
+    end
+  end
+end