feat(gallery): tag→gallery nav from modal chips (#5) + OR/exclude tag scope (#6a)

Cluster A, milestone #97. #5: clicking an image-modal tag chip's body now
closes the modal and opens the gallery filtered for that one tag (fresh
filter); ✕/kebab stay as the explicit remove/rename controls.

#6a (backend of OR/exclude filtering): gallery_service._apply_scope gains a
structured tag model — tag_or_groups (AND-of-OR: one EXISTS(tag_id IN group)
per group) + tag_exclude (NOT EXISTS(tag_id IN exclude)) — layered additively
on the existing tag_ids AND path so cursors/facets/deep-links are untouched.
Threaded through scroll/timeline/jump_cursor/facets/similar + facets common
dict; _require_single_filter rejects post_id combined with OR/exclude. API
parses tag_or (repeatable → one OR-group each) + tag_not (csv exclude).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

backend/app/api/gallery.py

@@ -37,16 +37,30 @@ def _parse_filters():
     """Parse the composable gallery filters from query args, returning
     ``(filters_dict, sort)``. Raises ValueError (→ 400) on malformed ids/dates.
 
-    `tag_id` accepts a single id or a comma-separated list (AND); `media` is
-    image|video; `sort` is newest|oldest; `platform` selects one platform
-    (or the UNSOURCED_PLATFORM sentinel); `untagged`/`no_artist` are boolean
-    flags; `date_from`/`date_to` are inclusive calendar-day bounds (date_to is
-    widened by a day so the whole day is covered by the service's half-open
-    `< date_to`)."""
+    The structured tag filter (#6) is AND-of-OR plus exclusions:
+    - `tag_id` accepts a single id or a comma-separated list — all ANDed
+      (the include common case; back-compat).
+    - `tag_or` is REPEATABLE; each instance is a comma-separated OR-group, and
+      the image must match at least one tag from EACH group (groups ANDed).
+    - `tag_not` is a comma-separated exclude list (image must carry none).
+
+    `media` is image|video; `sort` is newest|oldest; `platform` selects one
+    platform (or the UNSOURCED_PLATFORM sentinel); `untagged`/`no_artist` are
+    boolean flags; `date_from`/`date_to` are inclusive calendar-day bounds
+    (date_to is widened by a day so the whole day is covered by the service's
+    half-open `< date_to`)."""
     tag_raw = request.args.get("tag_id")
     tag_ids = (
         [int(x) for x in tag_raw.split(",") if x.strip()] if tag_raw else None
     ) or None
+    tag_or_groups = [
+        grp for raw in request.args.getlist("tag_or")
+        if (grp := [int(x) for x in raw.split(",") if x.strip()])
+    ] or None
+    not_raw = request.args.get("tag_not")
+    tag_exclude = (
+        [int(x) for x in not_raw.split(",") if x.strip()] if not_raw else None
+    ) or None
     post_id_raw = request.args.get("post_id")
     post_id = int(post_id_raw) if post_id_raw else None
     artist_id_raw = request.args.get("artist_id")
@@ -64,7 +78,9 @@ def _parse_filters():
         date_to += timedelta(days=1)  # inclusive of the date_to calendar day
     filters = {
         "tag_ids": tag_ids, "post_id": post_id, "artist_id": artist_id,
-        "media_type": media_type, "platform": platform,
+        "media_type": media_type,
+        "tag_or_groups": tag_or_groups, "tag_exclude": tag_exclude,
+        "platform": platform,
         "untagged": untagged, "no_artist": no_artist,
         "date_from": date_from, "date_to": date_to,
     }