Widen messages.signature to String(32) for Postgres

The column was declared String(8) but stores 16-char hex signatures (and can
hold the up-to-32-char packet_hash fallback from the LetsMesh normalizer).
SQLite never enforced the length, so the undersized definition went unnoticed
until db migrate-to-postgres rejected the data with
StringDataRightTruncation (varchar(8)). Widen the model column and add an
Alembic migration (batch_alter_table, so it applies on both SQLite and
Postgres). Verified end-to-end migrating a 1.2GB live SQLite DB into Postgres
(all 12 tables reconcile).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

alembic/versions/20260614_0915_d4e5f6a7b8c9_widen_message_signature.py

@@ -0,0 +1,42 @@
+"""widen messages.signature to 32 chars
+
+Revision ID: d4e5f6a7b8c9
+Revises: c3d4e5f6a7b8
+Create Date: 2026-06-14 09:15:00.000000+00:00
+
+The column was declared String(8) but actually stores 16-char hex signatures
+(and can hold the up-to-32-char packet_hash fallback). SQLite never enforced the
+length, so the undersized definition went unnoticed until a Postgres migration
+rejected the data (varchar(8)). Widen it to String(32).
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "d4e5f6a7b8c9"
+down_revision: Union[str, None] = "c3d4e5f6a7b8"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("messages", schema=None) as batch_op:
+        batch_op.alter_column(
+            "signature",
+            existing_type=sa.String(length=8),
+            type_=sa.String(length=32),
+            existing_nullable=True,
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("messages", schema=None) as batch_op:
+        batch_op.alter_column(
+            "signature",
+            existing_type=sa.String(length=32),
+            type_=sa.String(length=8),
+            existing_nullable=True,
+        )

src/meshcore_hub/common/models/message.py

@@ -21,7 +21,7 @@ class Message(Base, UUIDMixin, TimestampMixin):
         text: Message content
         path_len: Number of hops
         txt_type: Message type indicator
-        signature: Message signature (8 hex chars)
+        signature: Message signature (hex), or the packet_hash fallback
         snr: Signal-to-noise ratio
         sender_timestamp: Sender's timestamp
         received_at: When received by interface
@@ -60,7 +60,7 @@ class Message(Base, UUIDMixin, TimestampMixin):
         nullable=True,
     )
     signature: Mapped[Optional[str]] = mapped_column(
-        String(8),
+        String(32),
         nullable=True,
     )
     snr: Mapped[Optional[float]] = mapped_column(