refactor: migrate session and tree callers to SessionDAO and TreeDAO (#1235)

[marsof02]

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Part of Greenstand#1235 🦕

Summary

PR 3 of 6 for Greenstand#1235 — migrates session and current-tree callers off TreeTrackerDAO:

Stacks on #1 (branch chore/treetracker-dao-split-repos) and Greenstand#1303.
Merge after previous PRs, or review against that branch only.

TreeTrackerDAO is still used elsewhere; removed in a later PR.

Out of scope (follow-up PRs)

• PR 4 — planter (legacy) + tree capture
• PR 5 — location + sync/dashboard
• PR 6 — remove TreeTrackerDAO, split tests