perf: eliminate N+1 queries in plan range and meetings

[jlunder00]

Summary

GET /plan/range — O(days) → O(1) queries

• Added get_plans_for_range(conn, start_date, end_date) to db/pg_queries/plans.py
• Issues 6 queries total regardless of date range (was 5–6 per day): plan dates, tasks, recurring masters, exception rows, dependencies, acknowledgements, check-ins — all batched with BETWEEN $1 AND $2 or ANY($1) clauses
• Updated handler in api/routes/plan.py to call get_plans_for_range directly

POST /meetings/request + POST /meetings/{id}/accept — 3 N+1 loops removed

• Added get_users_by_usernames(conn, usernames) and get_users_by_ids(conn, ids) to db/pg_auth_queries.py
• Added get_connections_for_caller(conn, caller_id, target_ids) to db/pg_queries/scheduling.py
• Updated request_meeting handler: single batch lookup for usernames → ids, single batch connection check
• Updated accept_slot handler: single batch lookup for participant usernames

Test plan

• test_get_plan_range_uses_batch_query — patches get_plan to raise, confirms batch path used (requires DATABASE_URL on Pi)
• test_request_meeting_uses_batch_username_resolution — patches individual lookup to raise, confirms batch used
• test_accept_slot_uses_batch_user_lookup — patches individual lookup to raise, confirms batch used
• All existing plan + meetings tests pass (skipped here, require DATABASE_URL)