[alexeybe1kin]

Hunt finding memorygate-1 F28 (forgetting; Claude-written, one security review: no defect).

Before this change:

• Nothing deleted a pattern except a full reset.
• DELETE /observation/{id} removed only the row. Its lineage links, its id in other observations' related_observation_ids_json, and patterns built from it with their derived text all stayed.

Now:

• DELETE /pattern/{id} deletes a pattern of the calling agent (404 for another agent's) with its lineage links, and writes a pattern deletion receipt.
• Deleting an observation removes its links (each with a link receipt), its id in other observations and in patterns, and any pattern left with no observation (with its own pattern receipt).
• Recovery replay of observation and pattern receipts removes the same set from a restored backup. It is idempotent in either receipt order.
• The link and id-list helpers move to services/lineage_forgetting.py; the entity delete (fix: deleting an entity removes its history, events, edges, links and ids #63) uses them unchanged.

Limit: a pattern that still has other observations keeps its text.

Tests: tests/test_pattern_forgetting.py (3, all failing on main): route delete, another agent's 404, replay into a restored copy. Locally: every observation, pattern, recovery, deletion, forgetting, lineage, entity and reset test, 101 passed; ruff, types, sizes and package boundaries ok.