Add observer detail rows with SNR/path data and UI polish

- Add path_len column to event_observers table with Alembic migration
- Create shared observer_utils.py for building observer responses
- Extract SNR and path_len from LetsMesh uploads via normalizer
- Pass snr/path_len from all 4 handlers to add_event_observer()
- Add expandable observer detail sub-tables to messages and ads pages
- Rename Receivers column to Observers with updated i18n key
- Add satellite dish icon to observer badges and detail rows
- Add mobile card observer detail toggle
- Show full datetime tooltip on hover for relative times
- Hide Path column from advertisement observer tables (not populated)

alembic/versions/20260426_1052_a10dbca883a2_add_path_len_to_event_observers.py

@@ -0,0 +1,45 @@
+"""add path_len to event_observers
+
+Revision ID: a10dbca883a2
+Revises: b1c2d3e4f5a6
+Create Date: 2026-04-26 10:52:09.664958+00:00
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "a10dbca883a2"
+down_revision: Union[str, None] = "b1c2d3e4f5a6"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("event_observers", schema=None) as batch_op:
+        batch_op.add_column(sa.Column("path_len", sa.Integer(), nullable=True))
+        batch_op.drop_constraint(
+            batch_op.f("uq_event_receivers_hash_node"), type_="unique"
+        )
+        batch_op.create_unique_constraint(
+            "uq_event_observers_hash_node", ["event_hash", "observer_node_id"]
+        )
+
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("event_observers", schema=None) as batch_op:
+        batch_op.drop_constraint("uq_event_observers_hash_node", type_="unique")
+        batch_op.create_unique_constraint(
+            batch_op.f("uq_event_receivers_hash_node"),
+            ["event_hash", "observer_node_id"],
+        )
+        batch_op.drop_column("path_len")
+
+    # ### end Alembic commands ###