fix: evict expired entries from DbCache

[allamiro]

Fixes #283.

Problem

DbCache checked expiry on retrieval but never removed expired entries (long-standing TODO). With many users, encrypted databases of users who simply walked away accumulated in memory forever.

Change

store() now evicts all expired entries (one retain under the already-held write lock) before inserting. Eviction is opportunistic — any login or expiry refresh cleans the map — which avoids a background task while bounding growth to active users.

Testing

cargo test: 10/10 pass, including a new test that verifies an expired entry is dropped on the next store.

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Summary by cubic

Evicts expired entries from DbCache on store to stop unbounded growth from inactive users. Keeps memory bounded without a background task.

• Bug Fixes
  • Drop all expired entries under the existing write lock before inserting the new value (opportunistic eviction with retain).
  • Add a test verifying an expired entry is removed on the next store.

^{Written for commit 8dc021a. Summary will update on new commits.}