Rename "auto-filing" to "auto-assignment"

The feature that places a new Memory into the best-matching Topic was called "auto-filing" — a leftover from older filing-cabinet language that never fit. It's now auto-assignment everywhere it's visible: the capture confirmation reads "Assigned to [Topic]" (was "Filed to"), the Topic card's agent-placement badge reads "N auto-assigned" (was "N auto-filing"), the New Memory destination option is "Auto-assign" (was "Auto-file"), and the Topic-description helper text refers to auto-assigning. Behavior is unchanged — same embedding-similarity matching, same sticky human assignments. Under the hood the edge-function helpers and the topic-card metadata column were renamed to match (auto_filed → auto_assigned); a migration recreates the two topic-list RPCs with the new column name. The topic_origin provenance values (auto / human) are untouched.