Add configurable bucketing to TVL history endpoint

[Jayy4rl]

Context: Issue 135 adds TVL history with auto-bucketing. This issue makes the bucket strategy explicit and configurable.

Requirements:

• Add bucket query param to GET /api/v1/vaults/:contractId/tvl-history: "hour" | "day" | "week".
• Aggregate vault_tvl_snapshots by the chosen bucket using date_trunc.
• Return { bucket: string; avgTotalAssets; maxTotalAssets; minTotalAssets } per entry.

Acceptance criteria:

• ?bucket=day returns one entry per calendar day in the range.
• avgTotalAssets is the mean of all snapshot values within each bucket.