Add temporal decay scoring to semantic recall

[bug-ops]

Parent: #740 (P1)

Problem

Old memories rank equally with recent ones at the same similarity score. No recency bias.

Solution

Apply temporal decay: final_score = score * exp(-λ * age_days) where λ = ln(2) / half_life_days.

• Default half-life: 30 days
• Config: memory.semantic.temporal_decay_enabled (bool), memory.semantic.temporal_decay_half_life_days (u32)
• Requires storing timestamp in Qdrant payload (or SQLite join)

Affected crates

• zeph-memory

Acceptance criteria

• Older entries with equal similarity score rank lower
• Half-life configurable
• Disabled by default for backwards compatibility