refactor: rename the application package to docsgpt

The backend import package is now docsgpt, the name it will carry on PyPI;
application was far too generic to install into anyone's site-packages.
git mv plus a mechanical rewrite of every import, dotted string and path
reference: 734 Python files, the compose files, Dockerfile, workflows, docs,
setup scripts, devcontainer, k8s manifests, vscode config, pytest and coverage
config, .gitignore. Behaviour is unchanged.

Kept for one release:
- A top-level application package whose meta-path finder resolves
  application.x.y to the already-imported docsgpt.x.y object, so old imports
  and entry points (celery -A application.app.celery,
  uvicorn application.asgi:asgi_app) keep working with a FutureWarning.
- Celery registers every application.* task name as an alias of its
  docsgpt.* task on start-up, so messages queued by the previous release still
  run. The redbeat key prefix moves to redbeat:docsgpt:v2: so schedule entries
  the previous release wrote are left unread instead of firing twice.

The backend image builds from the repository root (docker build -f
docsgpt/Dockerfile .) so it can ship the alias package; a root .dockerignore
allow-lists docsgpt/ and application/ and keeps caches, local data, .env
files, the sample index files and the Dockerfile out. Compose and the image
workflows point at the new context.

docsgpt/storage/db/serialization.py

@@ -0,0 +1,93 @@
+"""JSON-safe coercion for PG-native Python types.
+
+Postgres (via psycopg) returns native Python types — ``uuid.UUID``,
+``datetime.datetime``/``datetime.date``, ``decimal.Decimal``, ``bytes``
+— that ``json.dumps`` rejects. This module is the single place those
+coercion rules live; everywhere else should call into it.
+
+Two interfaces with identical coverage:
+
+* :func:`coerce_pg_native` — recursive walk returning a JSON-safe copy.
+  Use when you need to inspect the dict yourself or pass it to a
+  serializer that doesn't accept a custom encoder (e.g. SQLAlchemy
+  parameter binding for a JSONB column).
+* :class:`PGNativeJSONEncoder` — ``JSONEncoder`` subclass. Use as
+  ``json.dumps(obj, cls=PGNativeJSONEncoder)`` for serialise-once flows
+  where the extra recursive walk is wasted work.
+
+Coercion rules:
+
+* ``UUID`` → canonical hex string.
+* ``datetime`` / ``date`` → ISO 8601 string.
+* ``Decimal`` → numeric string (preserves precision; ``float()`` would not).
+* ``bytes`` → base64 string. Lossless and universally JSON-safe;
+  prior code used UTF-8 with ``errors="replace"`` which silently
+  corrupted binary payloads (e.g. Gemini's ``thought_signature``).
+"""
+
+from __future__ import annotations
+
+import base64
+import binascii
+import json
+from datetime import date, datetime
+from decimal import Decimal
+from typing import Any
+from uuid import UUID
+
+
+def _coerce_scalar(obj: Any) -> Any:
+    if isinstance(obj, UUID):
+        return str(obj)
+    if isinstance(obj, (datetime, date)):
+        return obj.isoformat()
+    if isinstance(obj, Decimal):
+        return str(obj)
+    if isinstance(obj, bytes):
+        return base64.b64encode(obj).decode("ascii")
+    return obj
+
+
+def coerce_pg_native(obj: Any) -> Any:
+    """Recursively coerce PG-native types to JSON-safe equivalents.
+
+    Recurses into ``dict`` (stringifying keys, matching prior helper
+    behavior) and ``list``/``tuple`` (tuples flatten to lists since JSON
+    has no tuple type). Any other type passes through unchanged.
+    """
+    if isinstance(obj, dict):
+        return {str(k): coerce_pg_native(v) for k, v in obj.items()}
+    if isinstance(obj, (list, tuple)):
+        return [coerce_pg_native(v) for v in obj]
+    return _coerce_scalar(obj)
+
+
+def decode_base64_bytes(value: Any) -> Any:
+    """Reverse ``coerce_pg_native``'s bytes-to-base64 step.
+
+    Useful at egress points that need the original bytes back (e.g.
+    sending Gemini's ``thought_signature`` to the SDK on resume). Uses
+    ``validate=True`` so plain ASCII strings that happen to be
+    permissively decodable (e.g. ``"abcd"``) are not silently turned
+    into bytes — the original value passes through.
+    """
+    if isinstance(value, str):
+        try:
+            return base64.b64decode(value.encode("ascii"), validate=True)
+        except (binascii.Error, ValueError):
+            return value
+    return value
+
+
+class PGNativeJSONEncoder(json.JSONEncoder):
+    """``JSONEncoder`` covering UUID / datetime / date / Decimal / bytes.
+
+    Use as ``json.dumps(obj, cls=PGNativeJSONEncoder)``. Equivalent in
+    coverage to :func:`coerce_pg_native` but skips the eager walk.
+    """
+
+    def default(self, obj: Any) -> Any:
+        coerced = _coerce_scalar(obj)
+        if coerced is obj:
+            return super().default(obj)
+        return coerced