Add interaction logging and tighten model NOT NULL constraints

Log searches (article/category) and message-generation events to a new
interaction_log table, capturing IP address and User-Agent.

Also apply NOT NULL constraints to Contribution, SentMessage, FlickrUpload,
and ThumbnailCache fields that are always populated, and remove stale
continue_token references from category.html.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -92,6 +92,8 @@ those obtained via Flickr mail requests.
 - `flickr_uploads`: derived table built by `update_flickr_uploads.py` by
   matching Commons uploads to Flickr URLs
 - `thumbnail_cache`: cached Commons API thumbnail URLs (7-day TTL)
+- `interaction_log`: written by the web app to record searches and message
+  generation events (see below)
 
 **Key functions**:
 - `get_recent_commons_uploads()`: Loads uploads, filters by sent mail match,
@@ -115,6 +117,24 @@ Builds/updates `flickr_uploads` from `contributions` and links to
 - Extracts Flickr URL from contribution comment when present
 - Falls back to Commons `extmetadata.Credit` lookup when comment has no URL
 
+### Interaction Logging (`log_interaction`)
+
+The `log_interaction()` helper writes a row to `interaction_log` on each
+meaningful user action:
+
+- `"search_article"` – user submits a Wikipedia article search (page 1 only,
+  to avoid logging every pagination hit)
+- `"search_category"` – user submits a Wikipedia category search
+- `"generate_message"` – a non-free CC message is generated after clicking a photo
+
+Each row captures: Unix `timestamp`, `interaction_type`, `ip_address`
+(prefers `X-Forwarded-For` for proxy setups), `user_agent`, `query` (article
+title or category name), and optionally `flickr_url` / `wikipedia_url`.
+
+The table is created by `init_db()` (called via `python3 -c "from
+flickr_mail.database import init_db; init_db()"` or any of the maintenance
+scripts). The web app never calls `init_db()` itself.
+
 ### Category Search (`/category` route)
 
 Finds Wikipedia articles in a category that don't have images.