Digest cache grows without bound (no pruning) #97

Problem

CacheDao / CacheDatabase only ever does INSERT OR REPLACE and a full DELETE FROM cacheentry on "Clear Cache".

There is no automatic pruning of old entries by age, count, or size.

See:

• CacheDao.java (only insert, get*, clear, counts)
• DigestCache.java
• MainViewModel (diagnostics run on every launch)

Impact

• Database file grows forever on long-lived installs
• getUniqueCount() / diagnostics become slower over time
• "Clear Cache" is the only way to reclaim space (destructive)

Suggested

• Add a prune method (e.g. keep last N entries, or entries newer than X days)
• Run prune on app start / after sync (with a reasonable limit)
• Expose "Prune old cache" as a non-destructive menu option

Label: tech-debt,backlog