Add packet-groups API endpoint for deduplicated packet list

- New GroupedPacketRead/PacketReceptionInfo/GroupedPacketList schemas
- GET /api/v1/packet-groups: two-phase GROUP BY query, 7-day default
  window, lightweight Phase 2 (no raw_hex/decoded), role-aware cache
- GET /api/v1/packet-groups/{hash}: full reception list with path_hashes
  extracted from decoded.payload.decoded.pathHashes
- New (packet_hash, received_at) composite index via Alembic migration
- i18n keys for receptions, path, observer counts

https://claude.ai/code/session_01NH2rZzuHzasJj12SZeRhbJ

alembic/versions/20260613_0730_a1b2c3d4e5f6_add_packet_hash_received_at_index.py

@@ -0,0 +1,30 @@
+"""add packet_hash_received_at composite index
+
+Revision ID: a1b2c3d4e5f6
+Revises: e9f0c4079540
+Create Date: 2026-06-13 07:30:00.000000+00:00
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "a1b2c3d4e5f6"
+down_revision: Union[str, None] = "e9f0c4079540"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("raw_packets", schema=None) as batch_op:
+        batch_op.create_index(
+            "ix_raw_packets_packet_hash_received_at",
+            ["packet_hash", "received_at"],
+            unique=False,
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("raw_packets", schema=None) as batch_op:
+        batch_op.drop_index("ix_raw_packets_packet_hash_received_at")