Add at-least-once delivery + idempotent federation handler

- EventProcessor now recovers stuck "processing" activities back to
  "pending" after 5 minutes (handles process crashes)
- New ap_delivery_log table records successful inbox deliveries
- Federation delivery handler checks the log before sending, so
  retries skip already-delivered inboxes
- Together these give at-least-once + idempotent semantics

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/versions/s9q7n3o5p6_add_ap_delivery_log_table.py

@@ -0,0 +1,30 @@
+"""Add ap_delivery_log table for idempotent federation delivery
+
+Revision ID: s9q7n3o5p6
+Revises: r8p6m2n4o5
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "s9q7n3o5p6"
+down_revision = "r8p6m2n4o5"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        "ap_delivery_log",
+        sa.Column("id", sa.Integer, primary_key=True, autoincrement=True),
+        sa.Column("activity_id", sa.Integer, sa.ForeignKey("ap_activities.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("inbox_url", sa.String(512), nullable=False),
+        sa.Column("status_code", sa.Integer, nullable=True),
+        sa.Column("delivered_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+        sa.UniqueConstraint("activity_id", "inbox_url", name="uq_delivery_activity_inbox"),
+    )
+    op.create_index("ix_ap_delivery_activity", "ap_delivery_log", ["activity_id"])
+
+
+def downgrade():
+    op.drop_index("ix_ap_delivery_activity", table_name="ap_delivery_log")
+    op.drop_table("ap_delivery_log")