Add SQLite indexes for post-session queries

[Abhay512]

backend/sinks/storage.py creates thirteen tables and not one index. Every post-session query — reconstructing a ride, pulling a day of rankings, auditing signals — is a full table scan, and these tables grow by thousands of rows per session.

What to do

Add indexes in the existing additive-migration style (the _migrate_* methods show the pattern — CREATE INDEX IF NOT EXISTS, safe to run on an existing database):

• signals(ts), signals(underlying, ts)
• rankings(ts), rankings(underlying, ts)
• oneway_movers(underlying, entry_ts), oneway_movers(ts)
• whatever the other boards are actually queried by

Acceptance

• Migrations run cleanly on a fresh database
• Migrations run cleanly on an existing one and are idempotent
• EXPLAIN QUERY PLAN before/after in the PR for at least one query
• No write-path regression (these tables take one batch insert per minute)

Notes

Measure, do not guess — an index that is never used still costs write time. Say which queries you optimised for.